Federal and postal workers who are investing in their tax-deferred thrift savings program have earned more than $1 billion since it began in April 1987. The savings plan, the government's version of the 401(k) plan available to some private sector employees, is a voluntary part of the pension program, and it's now growing by $12 million each workday.

Employees who want to sign up for the savings plan -- or change their investment options -- have until the end of this month. Under new rules, workers can now invest their past and future contributions to the savings plan in any or all of the three options: a treasury securities fund that has a month-to-month guranteed rate of return, or the higher-risk stock and bond market options.

Most people hired since 1983 are in the Federal Employees Retirement System. FERS workers can earmark up to 10 percent of their pay to be put in the savings plan through payroll deductions. Those investing 5 percent or more get a 5 percent contribution from their employer, the government. Those who put in nothing still have accounts and get a

1 percent contribution from the government.

People under the old Civil Service Retirement System can invest up to 5 percent, but without any agency contribution. New rules lifting investment restrictions mean that both FERS and CSRS workers can invest in any or all of the three options.

Some savings plan facts:One million FERS employees have accounts. Fifty-six percent of them are contributing to their accounts, which are now worth an average of $7,177. The average account of those who contribute nothing is $746. About 460,000 CSRS employees have investment accounts, worth an average of $5,625. In both groups, the average contribution is slightly less than 5 percent of salary. It pays to invest early. A 20-year-old making $26,000 a year who invests 5 percent of pay (at 7 percent) will have $822,900 in his or her account at age 65. The same employee starting at age 30 would have $390,780, while that same worker who started investing at age 40 would have $175,760. Obviously, returns would depend on the actual rate of return, and would be less if the employee retired before age 65.

Federal officials expect that the removal of investment restrictions and the drop in the value of many stocks could prompt many employees -- especially younger workers who are willing to take risks -- to move into the stock or bond markets.Moonlighting

The Supreme Court yesterday denied a request by the National Treasury Employees Union to delay the effects of a new law that outlaws honoraria payments to federal workers who write or speak off duty.

The union had asked for the stay until the Circuit Court of Appeals rules on the issue. That court will hear arguments on Jan. 29. Meantime, the law -- and its $10,000 maximum penalty for violators -- is in effect, although many employees and politicians say it is unfair and perhaps unconstitutional.