After Jan. 1, persons eligible to receive food stamps will no longer have to pay for them as they do now. However, the number of stamps they receive will be reduced. The elimination of the purchase requirement is one of many changes required under new regulations of the federal Food Stamp Act of 1977.

According to U.S. Department of Agriculture figures, the regulations are expected to encourage 3 milliom more poor people to use food stamps while trimming 1 million people from the national food stamp rolls.

District of Columbia officials have said they expect the program to encourage about 140,000 people in 49,000 D.C. households, who are now receiving food stamp purchase cards but not using them, to benefit from food stamps once they become free. Only 91 percent of the eligible households use food stamps now because many people cannot afford to buy them, they said.

The regulations will be enforced in two stages. During stage one, eligible food stamp recipients will be mailed "authorization to participate" cards instead of the "authorization to purchase" cards they now receive. The cards will enable eligible persons to receive the bonus stamps they are entitled to.

The value of bonus stamps is determined by substracting what the person now pays for food stamps from the full value of the stamps now received. How much a person pays for food stamps is based on the household income. For example, if a person now pays $100 for $250 worth of food stamps, he will receive $150 in bonus stamps. All persons eligible for food stamps will receive bonus stamps after Jan.1.

Stage two, which will begin in March, changes the eligibility requirements for persons applying for food stamps. Some of those now eligible for food stamps may become ineligible after March when new guidelines will be applied to everyone.

The new eligibility regulations allow fewer deductions and household assets in computing net income than the old regulations did. Eight deductions were allowed before, but only three will be allowed under the new system.

For example, previously a family of four could deduct full day care costs, court-ordered alimony and support payments and tuition costs and fees when determining monthly net income. A maximum of $607 is allowed by law for a family of four.

The new regulations will increase that maximum to $640 for a family of four but will allow working people to subtract only 20 percent of theif gross income, $80 for child care and excessive household costs and a standard $65 deduction, which will be adjusted annually as the Consumer Price Index changes.

Maximum assets allowed will be limited to $1,750 per household, except in households where at least one person is 60 or older. These households are allowed maximum assets of $3,000.

Previously a family vehicle, regardless of value was exempt. Under the new rules a person who owns a vehicle with a market value of more than $4,500 is ineligible, unless it is used for self-employment.

The new regulations also apply to people receiving Aid to Families with Dependent Children and students.

Muriel Yager, food stamp outreach coordinator for the city, said area jurisdictions have set up information hotlines throughout the metropolitan area. Hotline numbers for D.C. are 724-4354, 724-4355; for Maryland, the toll free number is 1-800-492-0618; for Virginia 1-800-552-3431.

The Virginia hotline will not go into effect until Dec. 22.