The aim of this study was to evaluate and compare the plasma concentration of atrial natriuretic factor (ANF), K+, Na+, blood lactate, heart rate, and blood pressure in moderately trained women. Ten healthy women were studied on a cycle ergometer during 20 min of constant submaximal and maximal exercise, as well as during recovery. The ANF concentration was determined by radioimmunoassay. The results show that, except for Na+, all the other variables increased significantly with an increase in the duration and intensity of the exercise (P < 0.05, P < 0.001). In recovery, the values fell (P < 0.01, P < 0.001). Submaximal and maximal exercise both cause increases in ANF and this increase is due to the duration and intensity of exercise. However, maximal exercise, rather than submaximal exercise, is the major stimulus for the concentration of plasma ANF. ANF concentration may be a useful test for evaluating the releasing function of ANF in the heart.