With the spoonful-of-sugar approach PBS will once again offer doses of science, culture and theater when the new TV season gets under way this fall.

Devotees of PBS' unique dramatic touches will see a new Peter Wimsey (Edward Petherbridge) in three Dorothy L. Sayers' novels to begin airing on ''Mystery'' on Oct. 1. (PBS stations may vary the airdates.)

''Masterpiece Theatre'' returns to the '20s for a saga of ''The Bretts,'' a family of the ''theatah'' similar to the Barrymores or the Redgraves. No such similarity intended, the family is sufficiently high-strung and ego-motivated to make for some delicious viewing. The show debuts Oct. 4.

''Great Performances'' will boast a three-part series called ''The Tales of Hollywood,'' written by John O'Hara, Budd Schulberg and F. Scott Fitzgerald. Michelle Pfeiffer, Brian Kerwin and Christopher Lloyd are among the series' stars. The shows premiere Nov. 6 with John O'Hara's ''Natica Jackson,'' a somber story about what happens when a movie star falls for a noncelebrity.

But the most exciting new entry on PBS is the first comedy anthology created for public television titled ''Trying Times.'' Written by such renowned contemporary playwrights as Beth Henley, Christopher Durang and Bernard Slade, the six-part series explores the frustrations and occasional triumphs in coping with modern life.

Teri Garr plays a woman on the verge of mid-life crisis who must learn to drive a car, and Steven Wright portrays a 30-year-old perpetual student who has mastered the Punic Wars but doesn't know how to get a job. Other stars in the series include Rosanna Arquette, Julie Hagerty and Candice Bergen. Premiere date: Oct. 19.

''Degrassi Junior High,'' a special dramatic series aimed at junior high students, debuts Saturday. Unlike the teen fodder found in the movies, ''Degrassi'' takes a realistic look at adolescents and some of their problems like dating, peer pressure, gossip, drugs and friendship. This 26-part, half-hour series is created by the same team that produced the award-winning ''Kids of Degrasi Street'' which aired on the Disney Channel.

The 13-part ''Adams Chronicles,'' which originally aired in 1976, will be back for another visit beginning Sept. 24.

PBS is not about to forget science and has four new science-oriented series on tap this season. Dynamic MIT physicist Philip Morrison will host a six-part series about scientific inquiry -- how science knows what it knows and how it got that way. Morrison, an enthusiastic lecturer in the James Burke mode, takes viewers on a personal journey through scientific insight in this six-part ''The Ring of Truth'' premiering Oct. 20.

Pop naturalist David Attenborough will be back with ''The First Eden,'' a four-part program tracing man's impact on the environment of the Mediterranean from the first domestication of the beasts to his pollution of the land. The show begins airing Nov. 2.

''The Health Century'' commemorates the people who have made inroads into the mystery of medical science. The four-part series debuts Sept. 21.

''The Infinite Voyage'' will explore the unseen worlds of atoms and galaxies and ancient rivers. Premiering Oct. 28, the 12-part series will also air on commercial television.

The 200th anniversary of the Constitution will get a nod from PBS when it offers a four-part series about the impact of that esteemed document on the lives of Americans in ''We the People,'' debuting Sept. 22. Hosted by Peter Jennings, the program will be filmed in cities and towns where people are struggling with the very issues that are covered in the Constitution.

''America by Design,'' offers a personal perspective by architectural historian Spiro Kostof on the structures of our society. Not contenting himself with buildings, Kostof also examines the roadways, work places and the landscapes of America. The five-part series begins Sept. 28.