The scrotum of mature rats was immersed in a hot water bath (43?0.2℃) for 25 min once each month. One to seven days after the first treatment, the lysosomes, lipid droplets and mitochondria significantly increased in number in the sertoli cells. It also showed that heat treatment accelerated the mitochondria's degenerative changes and caused the mitochondria temporarily to multiply. The exfoliation of the sertoli cells into tubular lumen were often observed during seven to thirty days after the first heating, the exfoliated sertoli cells became more numerous after the second or the third heating.Examination with electron microscopy showed that heating did not damage the ultrustructure of the interstitial cells, but two to seven days after the first heating, the multiplication of collagen fibress was evident in the interstitial tissue. All the testes showed marked fibrosis after the second or third heating, when the rats were treated once a month, but of the animals were treated once sixty-five days, only an example of the testis showed a slight degree of the fibrosis after the second heating. These results suggest that the degree of the testis fibrosis is related to the duration of the interval between each successive heat treatment.