The objectives of the study was to investigate the effects on general practitioners' activities of a change in their remuneration system from a capitation-based system to a mixed fee-per-item and capitation-based system. It was our hypothesis that the effect of the change in remuneration varied between subgroups of doctors and patients as a result of the modifying effect of the doctors' age, sex, practice facilities, assistance, side jobs, and size of practice as well as patients' age, sex, and diagnostic group. The study was carried out as a follow-up with data collected from contact sheets completed by general practitioners in one period before a change in remuneration and two periods after. These data were supplemented by a questionnaire on doctors' characteristics as well as by health insurance data on population characteristics. The general increases in diagnostic and curative activities and reductions in referrals by general practitioners as a result of the change were found to be quite similar across subgroups of doctors and patients. While, total contact rates changed little, the sex of doctors showed a modifying effect: male doctors tended to increase their contact rates compared to female doctors. This tendency was most prominent among female patients with non-infectious diseases.