THE SPREAD OF DRUG USE HAS BEEN WIDELY STUDIED IN POPULATIONS OF ABUSERS, TYPICALLY HEROIN ADDICTS. MUCH LESS IS KNOWN ABOUT PATTERNS OF GROWTH OF OTHER DRUGS IN NORMAL POPULATIONS WHICH CONTAIN MANY DRUG USERS WHO DO NOT SEEM TO EXPERIENCE DIFFICULTIES. THIS STUDY ATTEMPTS TO HELP FILL THIS KNOWLEDGE GAP BY EXAMINING DRUG USE IN SCHOOL POPULATIONS (GRADES 7 THROUGH 12 AND 6 THROUGH 12) IN SAN MATEO COUNTY, CALIF., AND DUVAL COUNTY, FLA. DATA ARE DERIVED FROM SURVEYS OF STUDENTS TO DETERMINE DRUG USE FROM 1967 THROUGH 1977 IN SAN MATEO COUNTY AND 1973 THROUGH 1976 IN DUVAL COUNTY. SAN MATEO COUNTY'S SURVEYS ARE ACTUAL CENSUSES OF THEIR 7TH THROUGH 12TH GRADERS, INVOLVING FROM 18,000 TO 35,000 STUDENTS ANNUALLY. DUVAL COUNTY, ON THE OTHER HAND, USED A RANDOM SAMPLING SURVEY DESIGN INVOLVING 5,000 TO 10,000 OF ITS 6TH THROUGH 12TH GRADE STUDENTS ANNUALLY. THE CHIEF INTEREST IN THE SURVEY RESULTS IS THE ANNUAL PERCENTAGES OF STUDENTS IN EACH CLASS WHO STARTED USING A GIVEN DRUG, OR THE PERCENTAGES WHO WERE ACTIVE USERS OF THE DRUGS. THOUGH NOT EXACTLY COMPARABLE, THE TWO MEASURES OF DRUG USE REVEAL A COMMON TENDENCY IN THE TWO SCHOOL POPULATIONS; GROWTH OF NEW USE HAS BEEN SUDDEN AND SPORADIC FOR ENTIRE SCHOOL POPULATIONS RATHER THAN REGULAR AND CONSISTENTLY REPEATED IN SUCCEEDING CLASSES. IT IS ALSO CLEAR THAT USE SPREADS MORE QUICKLY DURING SOME YEARS THAN DURING OTHERS. SUCH BEHAVIOR SUGGESTS THAT NETWORKS OF FRIENDS AND PEERS ARE ABLE TO TRANSMIT NEW USE RAPIDLY IN SPITE OF CONSIDERABLE AGE DIFFERENCES. IN ADDITION, NEARLY ALL GROWTH CURVES FOR SAN MATEO COUNTY ARE DECLINING (EXCEPT FOR TWO OR THREE CLASSES FOR AMPHETAMINES USE) AND HAVE BEEN SINCE THE EARLY 1970S. DURING THIS SAME PERIOD, MOST OF DUVAL COUNTY CURVES WERE RISING. IT IS CONCLUDED THAT SAN MATEO'S 'EPIDEMIC' FOR ALL SUBTANCES WAS A PHENOMENON OF THE EARLY 1970'S, WHILE A SIMILAR PERIOD OF ACCELERATING GROWTH DID NOT OCCUR IN DUVAL UNTIL THE MID-1970S. USE OF MARIHUANA AND OTHER DRUGS APPEARS TO SPREAD IN THESE HIGH SCHOOL POPULATIONS IN ABOUT THE SAME WAY THAT HEROIN HAS SPREAD AMONG KNOWN TREATED ADDICTS--IN EPISODES OF SUDDEN RAPID GROWTH FOLLOWED BY PERIODS DURING WHICH LITTLE NEW USE OCCURS. TABULAR DATA AND REFERENCES ARE PROVIDED. (RCB)