A California ballot measure that would phase out the use of at least 25 pesticides--and potentially ban more than twice that number--could force major changes in the way food is grown around the nation.

The measure, which will appear on the Nov. 6 ballot as Proposition 128, would ban pesticides that cause cancer or birth defects if the chemicals are used on food. Produce and other food commodities from outside the state could not be sold in California if they were grown with these pesticides.

The rules are among the most controversial provisions of Proposition 128, the environmental initiative known by its backers as "Big Green," because they ban pesticides even if they contain only trace levels of carcinogens or reproductive toxins.

No one is certain how many chemicals eventually might be lost to agriculture because many have not been fully tested or analyzed. Clouding the picture even more are widely different interpretations of the initiative by the environmentalists who wrote it and the California Department of Food and Agriculture. If the measure is passed, the agency stands to lose some of its responsibilities for pesticide regulation to the California Department of Health Services.

"There's an ironic campaign by Food and Ag to make this initiative even more far reaching than it is," said Al Meyerhoff, a senior attorney for the Natural Resources Defense Council, an environmental group sponsoring the initiative. "I know that once this passes . . . they will reverse direction 180 degrees and claim that the initiative doesn't include anything."

Jim Wells, an assistant chief of the department's pest management division, said Meyerhoff is "desperate."

"I think the proponents--when they saw what the impact was--realized it would frighten people, and are trying to back away from it and say, 'No, no no, it won't cover that.' "

But even under the most narrow interpretation, the pesticide provisions would be far-reaching.

"You are essentially dictating to other states how they can and can't use pesticides," said Jack Moore, a former U.S. Environmental Protection Agency assistant administrator and president of the Irvine-based Institute for Evaluating Health Risks.

Supporters and opponents predict the initiative would spark renewed efforts in Congress to curtail states' rights on pesticide regulation. Congress has failed to muster enough votes to prohibit states from enacting tougher regulations than those set by the EPA.