By manipulating regulations, the Environmental Protection Agency -- not the Energy Department -- has guaranteed adequate amounts of unleaded gasoline the past few months.

EPA's prime intent -- as reflected by the Federal Register -- was not to regulate the nation's gas supply level per se. That's DOE's province. Rather, EPA was concerned that, without unleaded gas, new-car owners in a shortage period would use leaded gas and destroy the emission-control devices in their cars. That, in turn, would lead to the pollution that EPA is out to stop. It was a sort of "carom-shot" regulatory situation.

EPA got into this last year, an official of its fuels section explained, when it looked like there might be an overall shortage of gasoline. At the time, the agency was trying to slow auto pollution by requiring a cut in the amount of lead that refiners put into their regular gas.

It seems, the official said, that the more lead put into gasoline, the more gasoline a refiner can produce.

The regulatory "deal" that EPA designed was a rule that permitted refiners to continue putting the higher amount of lead in regular gas if they agreed to increase their unleaded gas production at the same time by 6 percent over the previous year. That went into effect Oct. 1, 1979.

It worked. There were increased supplies of both types of gasoline. Along in December, however, the supplies of unleaded gas began to exceed demand. By spring, the unleaded supply was so great that its wholesale price was down to below the price of regular, even though unleaded normally costs two cents more to make.

The reason, according to EPA, was an overall drop in gasoline sales which reflected a falloff in new-car sales and better gas mileage from the smaller models being sold. The public was driving as much but using less gas.

Faced with these facts, EPA has decided to drop its October "deal" temporarily -- at least the part requiring the 6 percent increase in unleaded production.