What do you do when you're a household word at 81 but know you can't go on forever?

If your name is Benjamin Spock, you pick a successor, and never mind those pea brains who mutter about conceit.

"Both Spock and I prefer not to use the word successor," says newly crowned Michael Rothenberg, who just returned from a two-week national tour at his mentor's side, "in the sense he's still very much alive.

"As Ben likes to put it," says the pediatrician and child psychiatrist at the University of Washington School of Medicine in Seattle, "I'm his 'future successor.' As I like to put it, I'm the same old Dr. Rothenberg."

Rothenberg, 58, who collaborated on the newly released update of the classic Dr. Spock's Baby and Child Care (Pocket Books, $4.95) was tapped after Spock sought lists of candidates from pediatrics professors on the East Coast, West Coast and in Chicago. As it turns out, Spock's second wife, Mary, brought Rothenberg to her husband's attention after reading the younger doctor's work on the effects of television violence on children.

Spock's search, says Rothenberg, who has three sons and two grandchildren, was "not cheeky or egotistical," but well-intentioned.

"I think Ben was motivated by the same sense of moral responsibility with which he does everything. He realized the book has been enormously helpful to an awful lot of folks," and took steps, says the Seattle doctor, to insure its continued existence.

The mantle, however, still sits uneasily on Rothenberg's shoulders. His colleagues are supportive, he says, but "not dancing in the streets. There's always professional jealousy in the world, and medicine is no exception. People say, 'Oh, here comes the new Dr. Spock,' things like that."

Rothenberg's designation commits him to writing -- with Spock, if possible, or alone -- the next two editions of the classic child-care book, now 40 years old and with 30 million copies in print. Future editions are to be spaced at 6- to 8-year intervals.

"Ten years, the interval up to now," says Rothenberg, "is too long, given the rapidity of pediatric knowledge exploding these days."

Rothenberg, who was once taught as a medical student by Spock at Western Reserve University in Cleveland, says the two baby docs hold many political and professional beliefs in common, although they were not previously friends or associates.

"While Ben was at the head of the peace marches in the '60s , I was 3,000 or 4,000 people back," laughs Rothenberg. He says their agreement that nuclear war is the single greatest threat to children today prompted them to devote a new section of their book to the subject.

In other areas, says Rothenberg, there has been little change in parents' biggest concerns over 40 years: "discipline in the preschool and early elementary school years, school problems and peer group problems for school-age children and sex and drugs in teens."

He and Spock, says Rothenberg, also share strong feelings about talking to parents in a "nonpatronizing, nonpompous" way. "We both feel strongly about speaking directly to parents as sensible people . . . because basically they are."

Rothenberg says he subscribes to the philosophy that has been a theme of Baby and Child Care for the last 40 years: "Trust yourself. You know more than you think you do."

Rothenberg's manner with parents probably owes a good deal to Spock's influence. While at Western Reserve, he not only advised his admiring student to get psychiatric training, but also, says Rothenberg, "made possible my first residency. He called friends in New York at the Albert Einstein School of Medicine and made it possible for me to get into the school's new psychiatry program . . . He had a profound influence on me."

About the only disagreement Rothenberg says they had in preparing the current revision (the fifth) of Baby and Child Care was over a statement from Spock's '76 edition that fat babies tend to get eczema more than thin babies.

"I never heard that before," says Rothenberg, who challenged the assertion. Spock displayed his source. Says Rothenberg, "We finally changed it to read: 'Some doctors feel that fat babies tend to get eczema more . . .' "

Rothenberg declines to comment about raising his own sons, now 19, 26 and 32, because they made him promise not to talk about them.

Rothenberg has been on the faculty of the University of Washington School of Medicine since 1967 and is a staff member of the Children's Orthopedic Hospital and Medical Center in Seattle. He's published over 60 articles in medical journals, including one titled "Is There a National Conspiracy Against Children in the United States?" -- about how our culture "idealizes and romanticizes kids, but gives them short shrift educationally and in other ways."

Meanwhile, Rothenberg is nothing if not enthusiastic about being tapped as heir to the Spock throne.

"Most of my interest in the last 10 years has been in public-health education, health maintenance and the prevention of illness . . . When Ben's offer came along, it was a dream I hadn't even dared to dream. Ben's book is in essence a public-education document for parents."