To quantify the educational activities and types of teachers that medical students had in third-year clerkships at community-based teaching hospitals. In October-November 1992, 201 students in third-year medical clerkships at nine community-based hospitals completed a log that recorded the primary activity, site, and educator and method of education (for teaching or supervised activities) for each 15-minute interval of a 24-hour day. Each hospital offered at least three of the clerkships studied: medicine, obstetrics-gynecology (ob-gyn), pediatrics, psychiatry, and surgery. Statistical comparisons of the clerkships were done with chi-square analysis and one-way analysis of variance. The students received 6.5 hours a day of teaching with an instructor and committed an additional 4.9 hours to clerkship-related learning. Nearly 75% of the teaching fell to full-time faculty members and residents. In just over half of their educational activities the students participated with other learners, such as residents. The clerkships did not differ significantly in the amounts of formal teaching given; however, medicine did significantly more informal teaching, and surgery and ob-gyn did significantly more supervised practice. This preliminary study quantified medical students' educational activities in 1992 during third-year clerkships and provides baseline data describing these activities and the educators involved. Some findings may not be replicable, however, with the increasing demands of full-time faculty members in inpatient and outpatient settings and the shifting emphases in how and where residents provide instruction. Another study such as this one would help assess the effects on medical education of changes in the health care environment.