WASHINGTON — In a letter of resignation, the chief of the Forest Service on Tuesday urged the Bush administration to "withstand political pressure" and leave in place rules that would bar road-building across some 60 million acres of federally owned land.

The chief, Michael P. Dombeck, was a primary architect of that plan and other conservation measures in a four-year tenure during which he led the agency through a time of tumultuous change.

Still, Dombeck, 52, used his departure to defend not only the roadless policy but also other Clinton initiatives, such as the protection of old-growth forests. Those plans put him in conflict with mining and timber interests. Virtually all are now under review by the Bush administration, which has said it will try to do more to promote development along with conservation on public lands.

"Please remember that the decisions you make through your tenure will have implications that last many generations," Dombeck wrote to Agriculture Secretary Ann Veneman, who oversees the Forest Service.

Dombeck's announcement came three days before a federal court in Idaho is scheduled to hear the first of several legal challenges to the roadless rule, which was made final during President Clinton's last week in office. In advance of the hearing, the new administration has chosen not to offer a substantive defense of the rule, and it has postponed its effective date for 60 days, until May 12, to allow time for further review.

The tactics have prompted speculation that the new administration might work with the plaintiffs, who include the state of Idaho and Boise Cascade, in an attempt to limit the effect of the rules, which would apply to nearly one-third of national forest lands. In his resignation letter Tuesday, the outgoing forest chief warned directly against such a course.

"Doing so," he wrote, "would undermine the most extensive multi-year environmental analysis in history, a process that included over 600 public meetings and generated 1.6 million comments, the overwhelming majority of which supported protecting roadless areas."

The rules would ban roadbuilding and most timber-cutting in the designated areas, but in practice their effect would reach much further, by effectively barring most off-road vehicles and new oil, gas and mining operations.

Dombeck, a fisheries biologist by training, was the 14th chief of Forest Service. He seemed mild-mannered in most public appearances, but that demeanor only partially masked what admirers saw as a passionate commitment to conservation and critics saw as hostility to the agency's traditional forest-management role.

During his tenure, Dombeck clashed often not only with timber and mining interests but also with some Western Republicans like Sen. Larry Craig of Idaho, who as chairman of a key forest subcommittee has been one of the timber industry's most forthright defenders. In a letter last July, Craig accused the forest service chief of having been both "arrogant" and "slightly delusional" in the outspoken way in which he had advocated the roadless plan in the face of industry and Western-state opposition.

In a written statement Tuesday, Craig said of Dombeck: "We have not always agreed on policies developed for the national forests during the Clinton administration." But still, the senator added: "I admire Mike's commitment to his principles and goals."

"I think he was the first person to make the Forest Service realize its role as a conservation agency rather than a timber agency," said Bill Meadows, president of the Wilderness Society, a conservation group, which feted Dombeck at a reception in Washington earlier this month.

"I'm really saddened by the fact that he is leaving just as the new administration is failing to defend his roadless policy," Meadows said. "He was such a hero, and I think the entire Forest Service changed under his leadership, so I think it will be a huge loss."

The Society of Professional Foresters, whose members include people employed by the forest-products industry, has also suggested that Dombeck has leaned too far in the direction of conservation.

The group's spokesman, Jeff Ghannam, said that it hoped the new administration "will select a leader who values the role forest management plays in ensuring the health and productivity of the nation's forests." Among Dombeck's achievements in the last four years has been a 65 percent increase in the agency's budget, with most of the new money dedicated to aggressive new efforts to help protect forests and communities against wildfire.

Dombeck also put the agency on a track that would set as a formal goal the conservation of old-growth forests, whose wood products are highly prized by the timber industry for their rareness and size. But while he issued instructions in January to spell include that goal in a formal Forest Service manual, the guidance remains unwritten for now, and could easily be reversed by a new chief.

Among candidates for that job, administration and congressional officials today named several senior Forest Service employees, including Dale Bosworth, the regional forester in Montana, and Lyle Laverty, the former regional forester in Montana who is now director of the agency's fire plan.

Other candidates were said to include Ellie Towns, the regional forester for Arizona and New Mexico, and Lynn Sprague, the former regional forester for California.

As a career employee, Dombeck had been protected through May from reassignment by the new administration. But Wood, his former aide, said that he had preferred to step down now, and to retire altogether from the agency.

"The job of chief is too difficult for someone to serve as a placeholder, and Dombeck wants to remain an advocate for conservation," Wood said.