Every day at the Treasury Department, letters and money arrive from guilt-ridden Americans who want their sins forgiven.

"This check for $1,300 is to make restitution of tools, leave days and other things I stole while I was in the Navy from '62 to '67," reads one letter.

"This money that was not reported on my income tax has been on my mind for several years," reads another. "I want to make restitution."

And: "Please accept this money for two postal stamps I reused."

The cash, checks and money orders are deposited in what since 1811 has been called the Conscience Fund, one of four special accounts created to receive donations from people who feel compelled to donate money to the government. The other three accounts receive money earmarked for retiring the national debt, for helping pay for the national defense, and for nothing in particular (designated by the government as "gifts").

"Some of the letters that come in here are real unusual," says Jean Whisonant, the Treasury employee who catalogues them all.

There's the St. Louis man who sends in pennies, nickels and dimes that he finds on streets because he considers them government property.

There's the Midwest gentleman who, every Fourth of July, sends a birthday card to Uncle Sam, encloses $10 or $20, and signs it simply "Your Nephew."

There's the oil-rich Texas woman who died in 1957 and left the interest on her bonds to the government; every year about $90,000 arrives from the executor of her estate, making her the most generous regular donor to the government.

Not all gifts are in cash. The GSA routinely sells cars, like the 1957 Cadillac a donor left last year, and land left in wills to Uncle Sam. Whisonant says that few donors are looking for recognition, and many Conscience Fund donors are, for obvious reasons, anonymous. A recent letter from North Dakota said simply, "This is money on interest that I owe the U.S. government." Enclosed was a check for $802.

In the last two years, Conscience Fund donations have dropped off. In fiscal year 1980, $125,990 was given, but only $75,593 was received in fiscal 1981 --less than was received in such banner years as 1950 ($370,285), 1959 ($283,473) and 1974 ($298,456). Receipts are also off this year--only $21,229 had been collected as of May 31, for the fiscal year that ends the last day of September.

Receipts are off in the public debt fund too. In 1980, that fund collected $830,661, but in 1981, only $288,135 passed over Whisonant's desk. For this fiscal year, only $24,634 had been recorded by last March, though the $90,000 from the estate of the generous Texas woman had not yet been recorded for this year.

The unrestricted gift receipts have also followed the downward swing of the economy, plunging from $2,274,576 in 1980 to $1,512,106 in 1981. Receipts thus far for fiscal 1982: $548,802.

With receipts down, maybe it's time for a telethon for Uncle Sam? Not a chance, says Whisonant: "Treasury has always contended that if the average citizen is honest and aboveboard with his taxes, that's all he should be expected to pay."