The costs in time and money of medical student education for family physicians in private practice is uncertain, with the literature containing conflicting reports. Questionnaires were mailed in 1992-93 to 56 primary care physicians who had taught third- or fourth-year students during the previous academic year in fulfillment of the students' required four-week family medicine preceptorships at the University of Missouri-Columbia School of Medicine. Forty-six physicians responded. The physicians had been preceptors for 74 students. Most of the physicians reported increases in time spent at work (mean of 46 minutes per day, standard deviation of 32 minutes) when a student was present in their practices. Only five noted decreases in billed charges. The preceptors were consistent in their indications that having students in their offices increased their time at work. This suggests that teaching medical students places a substantial temporal burden on private practitioners.