Changes in erythrocyte carbonic anhydrase (RBC-CA) activity due to physical exercise were investigated using a measuring apparatus based on the electrometric method. RBC-CA activities were slightly changed due to transient aerobic (20-min run) or anaerobic (200-m run) exercise in moderately trained subjects, but there were no fixed trends. In 3 weeks of weight training, there were no differences in RBC-CA activities between before training and after 1 day of recovery in both trained and untrained subjects. However, after 1 week of recovery, RBC-CA activities slightly decreased in both types of subjects. The trained subjects showed higher levels of RBC-CA activity than the untrained subjects before and after training. The rest values of RBC-CA activity were higher in trained subjects than in untrained subjects. It was evident that RBC-CA activities were higher in subjects (especially in long-distance runners, swimmers etc.) who had undergone long-term strenuous aerobic training.