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Summary 1. The present study was undertaken to investigate the effects of aerobic physical exercise on membrane function in mild essential hypertension. 2. Hypertensive patients were divided into an exercise group (n = 8) and a non-exercise (control) group (n = 8). Physical exercise within the intensity of the anaerobic threshold level was performed twice a week for 6 months. Membrane fluidity of erythrocytes was examined by means of electron paramagnetic resonance (EPR) and spin-labelling methods before and after the trial period in both groups. 3. After physical exercise, blood pressure decreased significantly. 4. Compared with the non-exercise group, in the exercise group both the order parameter (S) and the peak height ratio (ho /h-1 ) in the EPR spectra of erythrocytes were significantly reduced (S, 0.717 ± 0.004 vs 0.691 ± 0.008, respectively (n = 8), P < 0.05; ho /h-1 , 5.38 ± 0.06 vs 4.89 ± 0.06, respectively (n = 8), P < 0.05). These findings indicated that exercise increased membrane fluidity and improved the membrane microviscosity of erythrocytes. 5. There was no direct correlation between blood pressure reduction and the exercise-induced increase in membrane fluidity of erythrocytes. 6. In the non-exercise (control) group, blood pressure and membrane fluidity were not changed after a 6 month follow-up period. 7. The results show that aerobic physical exercise increased erythrocyte membrane fluidity and improved the rigidity of cell membranes in hypertensive patients. The improvement of rheological properties of erythrocytes may explain, in part, the cellular mechanisms for the beneficial effects of physical exercise in hypertension. [ABSTRACT FROM AUTHOR]