HAVANA — A professor at the University of Havana was saying the other day that beards are back in vogue in Cuba. But the explanation owes nothing to nostalgia for the revolutionary image of Fidel Castro and his stubble-chinned band of \o7 barbudos\f7 .

"Check the markets," the bewhiskered professor shrugged. "See if you can find any shaving cream."

A housewife in Havana's Playa neighborhood leaned forward conspiratorially, as if to whisper a favorite recipe. "Shampoo is very difficult to find," she said. "Lots of people mix their own: Water. Raw sugar. Detergent. It leaves a strange smell, but it works."

And a dentist recounted buying a pair of shoes on the two shopping days for such items she is allowed each month under Cuba's elaborate rationing system.

"The first day, I looked everywhere. I stood in lines for hours and finally came home empty-handed. I was exhausted," she said. "The second day I finally found a pair after hours and hours. They fit, but I don't even like them."

At bakeries and grocery stores, at pharmacies and pizzerias, the lines grow longer and the shortages more severe. Virtually everybody in Havana, a lovely, crumbling city of 2 million people, is feeling the pinch of what may be revolutionary Cuba's most severe economic crisis.

Canned foods, automotive parts and machinery from Eastern Europe already had grown scarce as former East Bloc allies turned their backs on socialism and on Cuba.

Now the shortages are closer to home. Cuban rum and beer have been added to the rationing card. Pork, as integral a part of Cubans' Christmas as is turkey to Americans' Thanksgiving, was hard to find in Havana during the holidays after Castro declared an "amnesty" for pigs.

Bananas, while not officially rationed, are often scarce in the capital. Eggs, a protein staple, had been difficult to find but still available. Then, with two days warning, the government announced a limit of four eggs per person per week, beginning Jan. 21.

Other monthly allotments are enough to keep food on the table, but barely. Residents of Havana receive five pounds of rice a month, 10 ounces of red beans, 20 ounces of lentils, a half-pound of cooking oil (when available), four pounds of sugar and three small cans of condensed milk.

Three 12-ounce portions of chicken are included in the rationing each month. One 12-ounce serving of beef per month is generally, but not always, offered.