The typewriters of the federal government are always busy. Secretaries work on them during the day and thieves work on them during the night.

Next to calculators, typewriters are the most frequently stolen pieces of government property, the General Services Administration says. Last year 500 expensive machines were stolen from the 460 federal office buildings in the Washington metropolitan area. At a replacement cost of between $900 and $1,000 per machine, that adds up to nearly half a million dollars.

The latest rash of typewriter thefts has plagued the Department of Agriculture's offices on Belcrest Road in Hyattsville. In four burglaries over the last six months, 30 machines have been stolen from Federal Center Building One, one of three offices in a privately owned complex that the GSA leases space in.

Seven typewriters, some calculators, a radio and syringes from the health unit were stolen in the most recent burglary Jan. 14. In December, 14 typewriters and other pieces of office equipment were taken. The thieves pried open a side door with a coat hanger, and used office chairs to cart IBM Selectric typewriters to the elevators that they had hotwired.

"Typewriter ripoffs are not that difficult," said John Jester, chief of operations for the Federal Protective Service, which is in charge of security in GSA-leased buildings. "It could be the couriers, it could be the cleaners--everybody blames the cleaners--it could be anybody."

The thefts are being investigated by the Federal Protective Service and the FBI.

Security at the USDA complex has been bolstered in the past six weeks, by replacing stairwell locks and taking other steps.

And the stolen machines have been replaced after weeks in which secretaries typed not-so-natty-looking letters on machines that in some offices were dubbed "the clunkers." (The temporary models "didn't have a correcting feature," one secretary fretted.) Because the government has no property insurance on typewriters, replacements must come out of each agency's budget.

"There's no insurance company big enough to insure the government," said GSA spokesman Richard Q. Vawter. "It's cheaper for us to replace typewriters one by one than it is to pay the premiums it would cost to insure them."

According to Jester, the GSA recovered over 100 stolen typewriters last year, thanks in large part to a program that matched the serial numbers of stolen machines with the serial numbers of machines brought to IBM for repair.