WHEN IT'S FREEZING cold in Washington, it is pleasantly warm -- sometimes hot -- in Rio de Janeiro. So when the mail brought a brochure offering a medical seminar in Brazil in February, my husband and I reserved two places right away. My husband could satisfy some CME (Continuing Medical Education) requirements if he attended the seminar, and we could have a week's vacation in the sunshine in a city whose very name seems to promise pleasure.

Price per person for the week -- $599 plus tax -- included airfare, airport transfers, deluxe hotel and huge tropical buffet breakfasts. After we paid the deposit for the trip, we were reminded of the "fuel surcharge" for charter passengers. The total cost was $1498 for both of us. (Regular round-trip economy airfare alone would have been $980 per person from New York.)

On a bitterly cold Saturday night, we departed Kennedy Airport on a chartered DC-10 for the 5,000-mile flight to Rio. The tour operator had offered this trip to many, varied groups. He had also filled the plane with childsize seats. The plane was so overcrowed it would have been hard to insinuate a healthy sardine. High and increasing fuel costs were the reasons given for stuffing 367 passengers into a DC-10 biult to seat 290. (In contrast, Varig, the Brazilian airline, uses a seating configuration of 30 in first class and 234 in economy on the same route.)

After an uncomfortable night, with inedible food and warnings not to drink Rio's water or we would surely get "tourista," we arrived at Rio's modern new airport in the morning sun, wearing the glassy gase of the jet-lagged.

The introduction to Rio is so visually exciting, jet lag vanishes. The ride in from the airport is one postcard vista after another. The raw beauty of the place makes you gasp. And it all looked familiar, perhaps because Rio has been a setting for the Hollywood films.

To reach our hotel the elegant Rio Sheraton, our bus rolled through the city alongside Rio's magnificent shoreline, past the world-famous beaches. It was not yet 9 a.m., but the sand was packed with throngs of people who from a distance looked like great armies of ants on long stretches of sugar.

My husband spent each morning listening to lectures on the New Coronary Artery Disease, the seminar topic. I was free to enjoy the pleasures of the pool every morning. We found time each day for some serious relaxing on the beaches. The ocean, clear blue sky and warm sun proved a fine RX for the relief of winter doldrums.

The Coriocas (residents of Rio) are obviously crazy about their beaches. They are on them a good part of the day and night. Across a wide avenue, an almost solid phalanx of modern high-rise buildings faces the beaches. Not plunk on the sands, as in Miami. There are no private beaches in Rio --

On the beach almost anything goes. Soccer games and kite flying, pick-pockets and peddlers, and campaigning politicians. Businessmen with briefcases met and close deals there. Crooks steal there. Muscle boys cavort there. The national uniform is a skimpy bikini, it seems. Every night an army of sanitation workers rakes the sand. But the beautiful beaches are polluted. On days when the undertow in not strong or the surf rough, you wade through garbage to reach a clean area to swim in.

No one seems to sit still long enough to let a muscle relax. Physical fitness is almost a passion here, a religion. Cariocas head for the beaches very early in the morning. The jog, run bikes on the mosaic pavements. The city has thoghtfully provided an assortment of sporting equipment for public use. There are playgrounds for children on the sand.

Each afternoon we tore ourselves away from the sybaritic beach life and went sightseeing. At Corcavade, we rode a red funicular through wet jungle up the mountain to what is surely one of the most magnificient views on the planet. That jungle reminds you of how much tropical wilderness the Cariocas had to tame to build their city in this spot. Another day we took the shaky cable cars to the top of Sugarloaf. One afternoon, to escape the hurly-burly of the city, we saild on a local ferry to nearby Paqueta Island.

While searching for Rio's past, we happened on the new Mayan-style cathedral. Near the cathedral we boarded the last open wooden trolley in the city. The trolley rolls over the Roman-style aqueduct built by the Colonial Portruguese in the 17th century, and winds through the hilly, restored residential community of Sants Teresa, a charming pocket or Rio's past.

We over-ate the good restaurants, sat at outdoor cafes, drank the delicious Brahma Chopp beer. We discovered that if we walked a few blocks back of the beaches, there were plenty of inexpensive, modern churrascaria -- restaurants which feature beef and shrimp grilled on a blazing hearth. A two-inch-thick filet mignon costs about $2. We added a salad of tender hearts of palm for an additional $2. Local wines were good and cheap. Hors d'oeuvres frequently included quail eggs, pate, marinated thinly-sliced fresh vegetables and the special relish that makes the sawdusty farofa palatable. We enjoyed a cafezinho frequently, just like the Carioca. Brazil's best coffee may be exported to the United States, but the coffee in Brazil tasted better than ours.

In Rio you are surrounded by shops selling Brazilian gem stones. Like nearly everyone else, we visited Sterns where the jewels are temptingly bear beautiful. We thought about buying one or two. However, when we learned the cost, we knew we could live without them. On the flight home many passangers had jewels to declare to Customs. Evidently some of the passengers had gone to Rio on this charter specifically to buy gems, which they may consider a hedge against inflation.

Officially, Carnival would start in two weeks, but who could wait? Not the Carioca. Everyone was churning up for Carnival. We could see preparations everywhere. Many streets were already festooned with glittering decorations. You could feel the holiday spirit mounting. Construction work banged and clattered day and night. I wondered how people could sleep in the uproar. Evidently the Carioca loves and needs noise, gets "high" on noise.

Rio is an international city, composed of many ethnic groups. Home for 8.5 million people, it is one of the world's most overcrowded and underplanned cities. Rio is only 23 degrees south of the equator, but the Carioca does not take an afternoon siesta. Rio's people are involved wholeheartedly in self improvement by education. Correspondence courses flourish, as well as night schools. Newsstands are heavy with learning aids. I wondered if the general obsession with self-improvement extended to plastic surgery and the desire to improve one's physical appearance. Plastic surgery is commonplace here.

In Rio, if you are from the United States, you are "American." If you are from Brazil, you are Brazilian. (I had assumed we were all Americans, North and South.) There are not many Cariocas who understand or speak English. Their language is Portuguese. The Cariocas tell you proudly that, despite the racial diversity in the city, Rio has never had a race riot or an unpleasant public incident involving race realtions.

Rio has been enjoying an unprecedented economic boom for the past 10 years. Planes leaving the city have had to enlarge their first-class sections. The visual evidence is in the million cars that clog Rio's roads and the many luxury apartments costing more than a half-million dollars.

Unfortunately, there is a dark side to the dazzling economic growth. While the upper and middle classes have been thriving, indulging themselves in fast cars and face lifts, the mass of the people has been hit by soaring inflation (more than 70 percent in 1979) and a continuing series of social upheavals. The majority of Cariocas have not been cut in on the new prosperity. One fourth of Rio's residents live in the notorious flvelas -- slum shanty towns.

Many years ago, the very poor were crowded off the level ground and pushed up in the crazily slated morros (walls), the steep slopes that back up Rio. Millions of dollars are paid for land suitable for housing. The cliffs are available to squatters because it is not economically feasible to build on them. The favelas are shanty towns of discarded cardboard and tin shacks, slums without running water, electricity or sanitary facilities. Garbage is rarely collected, if at all.

There are attempts to slow down the spread of the squatter shacks. Whole neighborhoods are resettled in public housing. But as fast as the poor are moved in to clean, new neighborhoods, their emptied shacks fill up with newcomers to the city who move in and take their places. Rio is a magnet for people from all over the world, and especially from the impoverished northern hinterlands of Brazil. They come seeking work and opportunity.

In our wanderings in the city, we noticed the large number of stores selling magic portions and spirit dolls. In this 95 percent Catholic city, many people still practive the voodo art of Macumba.

Although we tried not to drink the tap water, we did get sick with "tourista." The intestinal upset gave us a day of misery and discomfort.

By the end of the week, a secret corner of me admitted that I was grateful that I would be gone from Rio and at home in a quiet place at Carnival time. But we had enjoyed our week in the sun.