A House Agriculture subcommittee, bowing to White House and chemical industry wishes, yesterday turned aside environmentalists' objections and voted to extend, unchanged, the nation's basic pesticide control law for two years.

Dismayed representatives of a coalition of national environmental groups, who had agreed beforehand to a one-year extension of the law while amendments were drafted, walked out of the legislative markup session in protest.

The full Agriculture Committee is scheduled to take up the subcommittee's two-year extension of the Federal Insecticide, Fungicide and Rodenticide Act (FIFRA) next week.

Jay Feldman of the National Coalition Against the Misuse of Pesticides said that his group will make another effort to limit the extension to one year or to add a series of tightening amendments to it.

The subcommittee's rationale for making the extension for two years was that the new administrators of the Environmental Protection Agency, shaken by controversy over favoritism to industry, need that much time to determine whether and how the pesticide law should be altered.

Among EPA officials purged in the controversy, which has drawn the investigative attention of six congressional panels, was John A. Todhunter, the assistant administrator for toxic substances and pesticides, who was alleged to have undercut FIFRA enforcement.

Both the Reagan administration and the chemical industry had pressed the subcommittee for a simple two-year extension of FIFRA to preclude the changes sought by environmental groups to strengthen public-health protection and pesticide-testing and -registration procedures.

But subcommittee members indicated yesterday that they were just as concerned about their own political health. They suggested that the two-year extension was more palatable because by authorizing FIFRA without change until Sept. 30, 1985, they would avoid pesticide debate during 1984, a presidential and congressional election year.

Rep. Steve Gunderson (R-Wis.), who wanted a three-year extension, said he felt a one-year extension would "put a gun" at his head. Rep. Leon E. Panetta (D-Calif.), calling three years as bad as one, said he didn't think it appropriate to deal with such controversial issues during an election year.

Feldman and other coalition members were rankled over the subcommittee's quick action. "It's astounding, because we've been told to work within the process and that if we want respect and credibility we should work with the Agriculture Committee. Now, in effect, they have snubbed us," he said.

Rep. Pat Roberts (R-Kan.), one of the architects of the two-year approach, and Chairman George E. Brown Jr. (D-Calif.) insisted, however, that the subcommittee's "two-track plan" will assure continuity in FIFRA while allowing the panel to take time to explore more controversial changes.

The two-year extension approved yesterday authorizes $66.2 million for FIFRA activities in fiscal 1984 and continues, through 1987, a scientific advisory panel that counsels EPA on pesticide questions.