The first federal estimated tax payment for 1991 is due by April 15, an IRS spokesman said.

Since April 15 is both the estimated tax payment due date and the individual tax return due date, the IRS reminds taxpayers not to mail both in the same envelope, as they are to be sent to different addresses.Taxpayers receiving income from which no federal tax has been withheld, or from which an insufficient amount has been withheld by a third party, such as an employer, usually must make estimated tax payments.

Generally, you must make estimated tax payments if you expect to owe $500 or more in federal tax for 1991, after subtracting tax withheld and tax credits, and your tax withholdings and credits total less than 90 percent of the total tax shown on your 1991 tax return, or are less than 100 percent of the total tax shown on your 1990 tax return.

Use Form 1040-ES to report the payments. The form includes estimated tax payment vouchers as well as a worksheet to help determine the proper estimated tax payments.

More information on estimated tax payments is contained in the free IRS Publication 505, "Tax Withholding and Estimated Tax." This booklet, as well as Form 1040-ES, can be obtained at your local IRS office or by calling, toll-free, 800-TAX-FORM.