TITO GOBBI, the most celebrated of post-World War II Italian baritones, made his Washington debut Wednesday at age 62 with Puccini's "Tosca." Gobbi estimated that he has sung the role of the cruel Roman police chief, Scarpia, one of the meatiest baritone parts ever, an amazing 879 times. And his classic recorded partnership in the opera with Maria Callas at La Scala has been described as the single finest opera performance yet preserved.

But Gobbi did not sing Wednesday night, nor will he when the opera is repeated today. What brought him here for the first time was the restaging for the Washington Opera of his "Tosca" production done several years ago for the Chicago Lyric Theater.

After 43 years in opera, Gobbi's vocal career is hardly over. The Metropolitan has announced he will take on a new role in Verdi's "Don Carlos" next year. But he is having second thoughts about that engagement, not because the role would strain his vocal powers, but " because it is too low, and would never have been right for me."

Meanwhile, Sarah Cardwell is after Gobbi to do yet another Scarpia, next year in Boston. "I could still sing it," he said, making stroking gestures toward his throat, "but I don't want to spoil the memories."

So Gobbi concentrates increasingly on staging and on teaching. His circumstances are such that he need do nothing but loll around the pool at his villa near Rome or travel around in his Mercedes. But "I could never retire," says Gobbi. "I must keep the mind going as well as the body."

His career has been remarkable, both in longevity and consistency. Compare it, for instance, to the tragically abbreviated career of the magnetic Callas, whose voice was frayed by her mid-40s and who died last year at 53.

They made no less than seven complete opera recordings together, and it was with Gobbi in "Tosca" that Callas made her last appearance at the Met in 1965. Gobbi is one of the few male singers who could match her formidable dramatic talents, as well as her music ones.

You heard all about how difficult she could be, but that was not true with us. We were really very good friends. And when they called me from Paris about her death, it was hard to believe. She disappeared from life like a fairy tale."

Memories of his career are very much on Gobbi's mind these days because he is gathering material for an autobiography. And stories about Callas will play a prominent role in that story.

"I was actually there in Florence that moment in 1953 when she decided to go on that famous diet. the one that literally transformed her physically," Gobbi recalls.

At that time Callas was 30. Only a little less than six years into her Italian career, she was already established as one of the generation's major singers and was in Florence for the first of her two complete recordings of Donizetti's perilously difficult "Lucia di Lammermoor," a searing performance still available on the Seraphim label. It was her first collaboration on discs with Gobbi, and the conductor was the venerable Tullio Serafin, probably Callas' most important musical mentor.

Serafin realized he had a historic singer in Callas. But on stage she was so obese that she seemed a veritable middle of the stereo-type gorgeous voice emanating from a static physique rounder than tall, thus dashing hope for dramatic credibility. The Met actually auditioned her to sing in "Madame Butterfly" as early as 1948, but general manager Edward Johnson concluded that the young Callas, at 210 pounds, was visually unacceptable for the delicate young Butterfly.

"Maestro Serafin had been after her about the weight for some time," recalls Gobbi, "Somehow he sensed she was potentially as - how do you say - charismatic an actress as she was a singer, and he kept pressing the point.

"He finally succeeded during a lunch break that day in Florence when a few of us sat down for lunch at an outdoor restaurant. Callas has refused even to weigh herself. But while we were eating, the maestro spotted a scale on the sidewalk across the street, one of those, you know, that you put coins in.

"He said, 'Maria, I will pay for it if you will just go over there and climb on that scale.' To the surprise of everone she finally consented, and I was deputized to accompany her. Before weighing she took off everything heavy she could. I ended up holding her coat, her shoes, her handbag and even her pearls before she would get up that scale. I forget exactly what the number was, but she was very heavy. She said very little.

"I forgot about it until a while later in Rome, when I was getting out of my car one day. I heard a woman call at me from the other side of the street. She was opening her coat as if to show off her figure and I didn't recognize her until I crossed the street and realized it was Maria. I stammered, "You are so beautiful," and she replied with relish, "Tell me, Tito, you aren't flirting, are you?"

From then on Callas was one of the most physically striking women to command a musical stage.

Gobbi discounts the theory that the diet, in which Callas lost in excess of 75 pounds, contributed to her premature vocal decline. Nor was her singing affected, Gobbi thinks, by her subsequent life as a glamorous jet-setter whose marriage to Aristotle Onassis was set for early 1968, only to be delayed and then precluded eight months later when Jacqueline Kennedy became his wife.

"After that I went to her and tried to convinced her to sing with me again," Gobbi recalls. "If you have trouble with a few notes, just leave them out', I told her. But she finally decided not to."

Meanwhile, Gobbi's career continued at a pace rare for any singer at any age. A new recording of one of his favorite operas, Puccini's "Gianni Schicchi," was released only a few months ago; Gobbi was in the title role and the reviews were glowing.

He estimates that he has sung 136 roles in 100 operas. Many regard him as the preeminent Rigoletto of his times (he has sung it 374 times, including a distinguished recording with Callas under Serafin). His performance of the title role of Rossini's "The Barber of Seville" is equally renowned (he has sung it 372 times, including another recording with Callas).

Probably no other opera singer's career has been so fully documented on records (27 complete operas). One reason Gobbi has sustained such as career is that his voice was never the suavest one, and therefore it has been slower that most to show wear. His was never the plush baritone of the late Leonard Warren or of Robert Merrill. But in many of the major roles in Gobbi's range - Scarpia, Macbeth, Rigoletto, Iago - smooth sound is less important than those qualities in which Gobbi's so excells.

Despite his enormous range, Gobbi has always taken efforts to conserve his voice. "I have been a sportsman all my life, and I know that when I begin to feel pain, or to get tired, it is time to stop. The same is true with the voice."

Cornell McNeil, the Scarpia in this week's "Tosca," will be the fifth to sing the role under the direction of the most experienced Scarpia of them all. Gobbi acknowledges that "my Scarpia influences the person a lot. I have studied it deep down. I have dug up lots of questions. I have asked the questions, 'What is fantasy?' and 'What is real?' It, like all good operas, must be a kind of working together."

Should a baritone in one his "Tosca" productions suddenly fall ill, Gobbi is asked if he would step in and sing his 880th Scarpia. He seems to duck the question. "I doubt that I would take the place of a colleague suddenly. It would be better to cancel the performance."