Guidelines for the regional odd-even gasoline sales plan announced by Maryland Gov. Harry Hughes, Virginia Gov. John N. Dalton and District of Columbia Mayor Marion Barry are:

Vehicles with license plates ending with an even number can buy gasoline on even-numbered days of the month. Zero is considered an even number.

Vehicles with license plates ending in an odd number can buy gasoline on odd-numbered days of the month.

Cars with personalized tags or tags having only letters can purchase gasoline on even days.

Taxicabs in the District would be exempt from the plan. Maryland and Virginia taxicabs, however, must comply with the odd-even requirements.

Saturdays, Sundays, holidays and the 31st of each month would be considered 'open' days when anybody can purchase fuel.

Out-of-state cars not bearing Maryland, Virginia or District of Columbia tags would be permitted to buy gasoline any day.

Other exemptions would include emergency vehicles, funeral vehicles, cars driven by doctors or nurses, school buses, commercial trucks and buses, motorcycles, mopeds, construction and agricultural vehicles, van pool vehicles driven by persons who must travel a minimum of 75 miles a day or 400 miles a week; judges and jurors who must travel that same distance from home to court, and persons whose livelihood depends on driving no less than 75 miles a day or 400 a week. Most of the exemptions will require carrying a certificate still being designed.