Chronic, as well as acute, exercise increases circulating PRL in females. The response of males to repeated exercise, however, is unknown. The purpose of the present study was to examine the effects of acute and chronic exercise on plasma PRL levels in untrained males. Eight male subjects performed cycle ergometer exercise at 50% of their maximal oxygen uptake on 10 consecutive days. The subjects exercised in an environmental chamber maintained at 39 degrees C and 30% relative humidity. PRL levels were measured on days 1, 5, and 10 before exercise, and after 20 and 45 minutes of exercise. Acute exercise increased PRL levels. However, plasma levels did not significantly increase during exercise on days 5 and 10. This suggests a similarity in the PRL response to acute exercise in males and females, but a sex difference in the response to chronic exercise.