Semen was collected at weekly intervals for 3 wk before and 10 wk after the sauna exposure at 85 degrees C for 20 minutes. The numbers, morphology, ultrastructure, motility, viability and metabolism of the sperm was assessed. Sperm numbers fell within one wk and slowly returned to normal in 5 wk. The earliest ultrastructural change was swelling of the plasma membrane, followed by an increase in the number of immature forms and disorganization of the arrangement of the mitochondria. Motility, glucose utilization and lactic acid accumulation of the sperm rose temporarily immediately after sauna.