Blood pressure, heart rate, heart volume, electrocardiogram, plasma renin activity (PRA) and plasma angiotensin II and aldosterone concentrations were followed up in sixteen male long-distance runners during various training periods from January to September. During the most intense training period, the "track-running' period, the athletes had significant increases in all measured parameters of the renin-angiotensin-aldosterone (R-A-A) system when compared to the seven control subjects. In the middle of this period the PRA and angiotensin II concentrations of the athletes had increased 1.9- and 1.5-fold, respectively, from the initial values. The plasma aldosterone concentration increased both in the athletes and in the control group. The systolic blood pressure of the athletes was slightly but significantly increased at the same period as the most marked changes in the R-A-A system were found. In the athletes, positive correlations were found between the relative heart volumes in the roentgenograms and the electrocardiographic signs for both right and left ventricular size. This study demonstrates that during intense training runners may have long-lasting temporary increases in systolic blood pressure level with parallel changes in the components of the R-A-A system. However, the reason for this slight increase in the systolic blood pressure remains unclear.