The oldest son of Vice President Walter Mondale, 20 year old Theodore, has learned to live two very different lives.

By day, he is a motorcycle enthusiast whose basic concern is cutting seconds off his amateur racing time and performing his job as an instructor and salesman for a Prince George's County motorcycle dealership.

At night, he returns to the formal surroundings of Admiral's House, the government's posh, secret-service- protected vice presidential residence in Northwest Washington.

The shadow of his father's fame has often caused problems for the free and informed life style that Ted Mondale says he prefers. "It got to be a pain when we won," said Mondale. He quit his job last year at a Cleveland Park Giant Food Store because "old ladies would come into the store all day and talk to me about the election." When the television cameras begin showing up, "I knew it was time to get down the road."

Now, Mondale - who looks very much like his father except for his long blond hair - says he is happy with his relative obscurity working for Beltway Kawasaki in Lanham.

The obscurity Monday says he enjoys at work comes to an abrupt end when he returns home at night and is greeted by guards at the front gate.

Mondale, however, said he glad Secret Service agents are stationed around his home. He recalled a recent incident when his home became an armed camp of Secret Service agents wielding shotguns.

"I remember returning home and being greeted at the gate by a swarm of guards with rifles. They told me there was a repeat of a man on the ground with a shotgun."

He said he usually doesn't take such threats "that seriously" but, he got very concerned when Secret Service agents found a high-powered rifle on the front seat of a car nearby his home.

Because of the security, Mondale said he has really had to limit the number of friends he can bring home. "The Secret Service agents can really hassle people they aren't familiar with."

Mondale said a number of his friends have to have their identification checked when they try to get inside the gate in front of the Vice President's home.

Beer or wine parties are out now, he lanected, because he is afraid one of his friends might spill something on the rug. "My mother would kill me."

He said he can no longer "throw my brother up against the walls" because his mother now has an expensive collection of art throughout their home.

Mondale put off finishing college because of his interest in motorcycle racing. He said he eventually plans to return to Minnesota where he will attend a private college. He tried a brief stint at American University but found, "I couldn't relate."

He said he and the rest of the family "were pretty sure" President Carter was going to select his father the night before the decision was announced. That big change brought a number of small changes to the life of Theodore Mondale.

He said he realized he couldn't get into trouble. He said he was glad he never told the press he smoked marijuana like former President Gerald Ford's son did. "You get stigmatized ifk you say the wrong thing."

He recalled an incident last year when he was riding his motorcycle nearby his home and got stopped by police for a minor violation. "My name went over the police radio and one of the television stations picked it up. The next thing I knew, television cameras were everywhere."

Mondale said he has been motorcycle crazy since he was 12 year old. He began working for the motorcycle dealership in Lanham "not for the money," but because he could get motorcycle parts at a discount. He said he has gone into debt since he took the job.

Thedore Mondale - who carries the same name as his grandfather who gave up farming in 1911 to become a minister - says he wakes up early in the morning and conditions himself by riding for hours on his motorcycle.

For now, issues such as the Panama Canal or the President's energy policy have taken a back seat to his number one passion.

But in the future, Mondale says he will probably choose politics as a career because: "it is my best shot." Mondale said he has already gotten some experience campaigning for his father.

Mondale said he has gotten a chance to meet many members of the President's Cabinet. He said he really likes Bert Lance, the former budget director, because he "really talked to me and he was a nice guy. Bert really blew it."

Mondale said his greatest disappointment is that the Watergate backlash in the White House has prevented him, as well as the rest of the family, from riding along with his father when he goes on trips.

"It was all because Julie Nixon would have Air Force One fird up anytime she wanted to go to Miami. It would cost around $10,000 just to have the plane sent there."

He also does not think it is right that he can not get a job on Capitol Hill. "Just because I'm the Vice President's son, I can't get a job . . . It isn't fair . . . Since Watergate, everybody is looking for any kind of dirt they can find."