Word got out early yesterday morning that Prince Georges County Executive Lawrence Hogan was off on some mysterious mission, and for the rest of the day Upper Marlboro was buzzing with speculation about where he was and why he had gone.

Hogan's aides verified that he had left on an early morning flight, but even his son, Larry Jr., who is also a top assistant, said he was baffled by his father's disappearance.

All of this confusion, however, did not touch Hogan. He spent the afternoon sitting calmly between the executives of two diet soft drink companies at the Calorie Control Council convention in Atlanta, adding $1,000 in speaking fees to his $51,000 county salary.

"This isn't a junket," said a relaxed Hogan. "I don't feel bad at all being here. The job (as executive) is all-consuming and it keeps me there at times until 10:30 at night. And I don't get overtime."

The calorie convention, devoted to the promotion of artificial sweeteners such as saccharin, consisted of workshops, slide shows and speeches. After every other speech, the convention delegates were offered "diet breaks," during which they could partake freely of Shasta diet orange soda and Dad's diet root beer.

Hogan stuck to Fesca, his favorite soft drink, but did not confine himself to other low-calorie fare when it came to convention meals. After eating a slice of strawberry shortcake loaded with whipped cream, Hogan said he saw "nothing wrong with saccharin, unless you're a rat. If you're in a laboratory, you're in trouble."

In addition to the speaking fee, the council, which is a saccharin lobbying group, paid Hogan's expenses, including a deluxe hotel room in Atlanta's Omni International Hotel.

Sandwiched between a speech on "Changing Shares of American's Stomach," and a cocktail reception featuring diet drinks, Hogan's remarks drew polite applause but few laughs for his jokes.

"He wasn't particularly impressive to me. I found his disappointing and repetitive," said an executive of a New York dietary food firm.

In his 40-minute speech, Hogan criticized regulatory agencies and encouraged businessmen to actively support their political candidates "or else you won't halt the inexorable march of this country toward socialism."

He told the 90 businessmen on the council, "You can't afford to let the Ralph Naders of the world and his lemmings write the regulations that are going to put you out of business."

Hogan, it turns out, was the council's third choice as featured speaker. He was selected after a senator and congressman said they could not make it.

"There we were hung up, I was looking for a speaker who was knowledgeable and could speak on looking ahead on the marketing situation," said Larry C. Davenport, one of the convention's directors.

Hogan said he accepts about four or five speaking engagements every year, "and I've done so for the last 20 years."

However, he said he agreed to come to Atlanta because he was expecting to continue on to another city after the convention to interview a candidate for county police chief. That interview was canceled because of schedule conflicts. Hogan said.

His trip to Atlanta, Hogan said, in no way meant that he was neglecting his duties back in Maryland. "I resent the hell out of that," he snapped. "I've talked to the office four or five times today, and I probably won't stay the night."

In fact, said Hogan, "I haven't even been out of this hotel."