";Those who swim think it';s the healthiest thing in the world,"; said 62 year old Nina Mikhailova, as she sits sweating buckets in the Finnish sauna where these enthusiasts warm up before their brief communion with the icy water (around 0.5 C). ";Those who don';t do it think it';s absolutely the worst, most senseless activity.";

And these are the sane ones.

The real extremists, the ";Neva Walruses,"; shun the preliminary sauna, undressing and jumping directly into the river in -20 C weather.

This extraordinary winter ritual occurs daily in the Neva by the Peter and Paul Fortress, near the Lenin Stadium, in Park Pobedy and at several other locations in St Petersburg.

Out on the frozen Neva, Yuri Vasiliyev and fellow Walrus Sergei Devchenko prepare their hole by breaking up the thick layer of ice which formed overnight. ";I do this so I don';t get rusty,"; said Vasiliev, 66, a robust pensioner who goes in every day except Saturday and Sunday.

There are about 500 practising Walruses in St Petersburg, about 50,000 in all of Russia.

The official Walrus club in the city, Nevsky Morzhi, was closed after their hut on the beach of the Peter and Paul Fortress was removed to make room for the Goodwill Games volleyball competitions held last summer.

The Neva Walruses remain a close knit community, socializing around their holes and espousing the medicinal, emotional and spiritual rewards of their peculiar common hobby.

";A lot is written about how healthy this is,"; said Vasiliev.

";When you first go in there';s a little shock";, he bends down and immediately begins to huff and puff dramatically into the cold air.

";I wouldn';t recommend it to anyone with a heart condition,"; said Anna Vejlens, a doctor at the American Medical Center.

";Jumping in cold water won';t make you healthy. It';s the other way around: if you can take it you';re already in fine shape.";

Russian doctors are more enthusiastic. One study published in the Russian scientific journal Khimia i Zhizn showed that ";Walruses"; get sick 10% less often than the general population.

Following the logic that cold water immersion fortifies the immune system, parents who swim themselves start dunking their babies when they';re only a few days old.

In a small hole in the ice of Sredny Lake last week a 13 day old girl was dipped, toes first, into the water; crying going in, shocked into silence on the way out.

";I swam first when I was four years old,"; said Seva, a part time Walrus and former bass player with the rock group Aquarium.

Nobody who';s tried it would refute the physically ecstatic effects that follow the initial shock.

Jumping into icy water causes blood vessels to contract quickly while heart rate increases, resulting in a sudden jump in blood pressure.

The physiological shock releases adrenaline and endorphins, which causes euphoria. ";The popularity of winter swim-ming is easy to explain,"; wrote Dr L I Korotkova author of the Khimia i Zhizn study. ";Icy water is a magnificent, non-narcotic stimulant for the whole body, causing emotions of vivacity and joy.";

But don';t get too carried away. It';s possible to faint from the shock and then drown.

Walruses never swim alone . Two minutes in the water is the absolute safe maximum.

After 12 minutes in freezing water a person can lose consciousness. Within 18 minutes he';ll be dead.

Women lose heat in the water slower than men, but they also recuperate more slowly.

While fat people stand a better chance through insulation children lose heat twice as fast as adults and should not linger in the water. For some Walruses the experience is a spiritual experience.

";Why am I here?"; asked Walrus Yuri Sultanov, jumping up and down to keep warm after his swim. ";It';s my way of praising the world, the river, the Saints, of praising God.

";Look around, it';s beautiful here,"; he said, sweeping his arm to take in the sparkling river ice, the clear blue sky and the bright green facade of the Winter Palace across the Neva.

As he talked his swimming shorts, lying on a newspaper on the ice, froze stiff and crinkled as he packed them away.

Photo: Yelena Yakovleva. (Large jpg - 105K)