The Pentagon is quietly upgrading its ability to wage war with nerve gas, a weapon that can cause agonizing death from spasms and convulsions if only a drop or two gets on a person's skin.

The Navy is spending $1.5 million this year to resume research on a bomb, code-named "Big Eye," that would spread the gas over a square mile. The Army is spending $2.7 million to continue research on a "binary" artillery shell that would mix two harmless chemicals in flight to form nerve gas.

Work on the bomb had been canceled in 1969 - the year the United States ended its germ warfare program, and one year after nerve gas wafted 30 miles from a test site in Utah and killed 6,000 sheep.

A "de-emphasis" followed, including the decision to phase out the Army's Chemical Corps, which dealt with defense against gas as well as weapons development.

The Army now has reversed that decision. The reason it cites is a growth in the Soviet Union's chemical warfare capability, although both nations have signed a treaty renouncing its first use.

"We underestimated the threat and it got us in trouble," said a chemical warfare expect. "We woke up in 1975 and decided it was a bad story. The Soviets were pushing their capability. Our training wasn't being done and our soldiers weren't capable of surviving in chemical warfare."

Officials cite reports from Russian defectors that the political decision to use chemical warfare already has been made, and Soviet commanders can issue the orders on their own. In the United States, the order would have to come from the President.

Chemical warfare experts talk in a chilling way about possible use of gas mines, missiles, bombs and shells as defensive and offensive weapons.

Nevertheless, most of the thrust of this year's program - $147 million out of a total of $186 million - is defensive, with priority on overgarments that would protect troops from Russian nerve gas. U.S. units do not now have such equipment.

Officials say production of te binary shell would make the stockpile of chemical weapons, kept mostly at U.S. depots, more safe. Congress vetoed $15.9 million to start work on production facilities for the shells in the past two years, and former President Ford took the request out of the budget now under consideration.