If you think you are in a high tax bracket, consider the plight of the government retiree's widow who just got her April survivor's annuity check. The government withheld $390 from her $400 payment.

Or how about the retired civil servant in Massachusetts who got his $700 April pension check this week, only to discover that Uncle Sam kept $500 of it for taxes?

Then there is the Rhode Island man who, last January, asked the government to withhold $330 from his annuity check beginning this month. The check he just got had more than $600 withheld, nearly double the amount he thought he requested.

Those horror stories, unfortunately, are not isolated events.

Many government retirees--there are 60,000 of them in the Washington area--are in a state of shock after looking at their April checks which started arriving here last Friday. This is what happened:

Congress last year changed the tax law to begin automatically withholding tax money from government retiree annuity checks. The withholding started this month.

Early this year the Office of Personnel Management hired Westinghouse Information Services in Iowa City to help it contact more than 2 million government retirees, advising them of the new law and giving them a tax-withholding election form to fill out.

The forms--many people never got them or got them too late to meet a mid-February deadline--gave retirees the option of designating how much tax they wanted withheld, or the option to continuing to pay taxes themselves quarterly.

Retirees who were already having tax withheld (about one in every five) did not have to return the forms. Others who failed to return the forms, however, were subject to a standard withholding rate at the rate of a married couple with three deductions.

The OPM--which has been swamped with complaints--says that many retirees apparently didn't understand the new withholding system. Some, OPM says, put down the amount they wanted deducted for the entire year. But that amount is being taken out each month, and will be until the retiree "corrects" it.

Others, OPM said yesterday, put down a dollar amount to be withheld monthly, unaware that that amount would be added to the amount to be deducted based on the number of exemptions they claim.

The OPM says people didn't read the instructions carefully, but many retirees say the whole program was mishandled. Whatever the reasons for the extra-heavy deductions, a lot of people have been shortchanged this month.

Members of Congress from the Washington area have been inundated with calls from angry, frustrated retirees. The National Association of Retired Federal Employees says it has been hearing from retirees all over the country, who are wondering what happened to their annuity checks.

If you have a problem with your check, this is what to do: If you want no money withheld, you can write the OPM a letter. They will stop the deductions as soon as possible and you can pay taxes quarterly. If you want the amount being withheld changed, you must get a W4A form from the OPM. Either way, write to this address: OPM Allotment Section, P.O. Box 961, Washington, D.C. 20044. OPM today will set up two special information numbers today for retirees with questions about their checks. Those numbers are 632-0860, or 632-4687.