We have previously demonstrated that scrotal hyperthermia induce Leydig cell (LC) damage in short-term. The objectives of this pilot study were to investigate morphological changes and regulation of steroidogenesis on LC in long-term and the time of observation were extended to investigate whether the LC would eventually make a recovery after scrotal hyperthermia. The rats were randomly allotted into one of four groups: A (control), B (70 days after scrotal hyperthermia), C (105 days after scrotal hyperthermia), D (140 days after scrotal hyperthermia); each group contain seven animals. Scrotal hyperthermia was carried out in a thermostatically controlled water bath at 43A degrees C for 30 min once daily for six consecutive days. Control rats were treated in the same way, except the testes were immersed in a water bath maintained at 22A degrees C. Hyperthermia applied rats were sacrificed under 50 mg/kg ketamine anaesthesia after 70, 105 and 140 days, and biopsy materials of testes were obtained for light and electron microscopic examinations. Morphologically normal and the number of testosterone positive LC was significantly higher in 140 days after last heat than all other heat treatment groups. In heat treated groups, a dilated smooth endoplasmic reticulum, swollen mitochondria, and vanished mitochondrial cristae were observed. In the 140 days after scrotal hyperthermia, the severities of degenerative changes of LC were less than that observed in the other heat treated groups. We conclude that, scrotal hyperthermia cause morphological damaging and impaired steroidogenesis in LC and recovery of these findings were noted first time in 140 days after the last heat treatment.