A study examined both cognitive and activity rhythms of lessons in a variety of disciplines and teaching styles in the medical school curriculum among 20 faculty members at Ben Gurion University, Israel. Studied were changes which occurred in the rhythms following a teacher training program, and interrelationship among rhythms and size of learning groups. Teachers involved in the study took part in a 4-day workshop, based on microteaching techniques, which aimed at improving teachers' instructional behavior in two dimensions: (1) to replace the traditional teaching modality of lecturing by a pattern which elicits students' self-initiated verbalization; and (2) to use and stimulate the students to use a higher cognitive level of verbal exchange during the lesson. Results showed a significant rise in activity and cognitive levels in teachers' classes following the training workshop; however, the educational gain from the training varied considerably in the different learning groups according to size. Three distinct rhythms of lessons--dramatic rhythm, dynamic rhythm, and static rhythm--were disclosed for both cognitive and activity parameters. There was a prevalence of "static rhythm" after the workshop in all group sizes. (JM)