Times have changed at the border between Greece and Yugoslavia.

Twenty-five years ago it was a formidable barrier, closed to prevent Greece's defeated Communist armies from fleeing north.

Today it is open and thriving. Now, the Yugoslavs are heading south.

Every weekend they spill across the border, well-dressed, driving private cars. One of the most talked about phenomena in this northern Greek city, known to Greeks as Thessaloniki, is the alluence Yugoslav.

They come here, and to the Florina area of Greece's Macedonia region, as tourists, as young lovers, but more often, on Fridays and Saturdays, to shop.

Last year some 700,000 Yugoslavs crossed the border of thei Communist-ruled nation, loaded with American dollars, French francs and West German marks.

"They're spending a fortune," said one diplomatic official, "But it's impossible to compile figures. There is no currency control in Yugolsavia, and every Yugoslav has both a dinar (the Yugoslav currency) and a foreign currency account. But, presuming that they all spend one, two, three hundred dollars on each shopping expedition, this is an extraordinary amount."

Figurs of the Greek National Tourist Organization indicate that in 1976 Yugoslav tourists spent $11 million in Greece. But officials caution that the real figure is much higher.

"This accounts for transport, lodging," said one tourist official, "not for all their shopping sprees."

They crowd the hotels of one of Salonika's main shopping areas, the bustling Egnatia Street. In the fashionable restaurants one hears a babble of Slavic today.

The Yugoslavs has bought up thousands of cassettes, radios, and the resplendent, often garish crystal chandeliers that are a trademark of this city. They also buy textiles, shoes and leather products, fashion items from Salonika's chic boutiques.

In the past, they have come individually, not in organized tour groups. But, during Greece's annual February sales period, an enterprising Yugoslav tour operator organized hundreds of busloads from Skoplje and Bitoj, to do their shopping in Greece.

"I've been in business for two years," said Yiannis Alevropoulos, 24. "They could make me a millionarie."

Outside his tiny shop on Egnatia, an American flag is emblazoned by flashing lights. Alevropoulos's shop, "Unisex," specialized in American blue jeans.

"Eighty per cent of my foreign customers are form Yugoslavia," he said. "On weekdays, I take in about $1,000. When the Yugoslavs come on Friday and Saturday, I gross three times that amount."

Alevropoulos's American and Italian blue jeans go for $33 a pair. They come in all shapes and sizes, as do his Yugoslav buyers.

A sign in his window announces that Macedonian, the language of Yugoslavia's southermost republic, is spoken. The language is now a requirement for all of his clerks.

In the large department stores of Glaoudatos and Dimitriadis, price tags are in both Greek drachmas and Yugoslav dinars. Clerks speaking Macedonian are given preference in hiring. Some shops advertise a 10 per cent discount for the free-spending Yugoslavs.

"During last summer's economic crisis," said merchant Stavros Kassiadis, "We were saved . . . we got rid of our entire stock of miniskirts to the visiting Yugoslavs."

The queues have become so massive at the border crossing points of Idomeni and Evzoni, 52 miles from here, that Yugoslav officials are now negotiating with Greek authorities to open additional frontier crossings. They are also urging the Athens government to eliminate visas. In Europe Yugoslavia requires visas for only Albania, Spain and Greece.

"It's become a fad," said one official. "Greece is now the 'in' place. (President) Tito's been here, (Greek Prime Minister Constantine) Karamanlis has visited Yugoslavia."

"It shows how things are changing in Europe and the eastern Mediterranean today," said one analyst. "The threat is now considered to be from the east, from NATO partner Turkey. It's no longer from the north."