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This study investigated the effects of high temperatures on male germ cell development and epididymal sperm motility of laboratory mice. In Experiment 1, adult males (n=16) were exposed to whole-body heat of 37-38 degrees C for 8h day(-1) for 3 consecutive days, whereas controls (n=4) were left at 23-24 degrees C. In Experiment 2, adult mice (n=6) were exposed to 37-38 degrees C for a single 8-h period with controls (n=6) left at 23-24 degrees C. Experiment 2 was conducted as a continuation of previous study that showed changes in spermatozoa 16h after exposure to heat of 37-38 degrees C for 8h day(-1) for 3 consecutive days. In the present study, in Experiment 1, high temperature reduced testes weights 16h and 14 days after exposure, whereas by Day 21 testes weights were similar to those in the control group (P=0.18). At 16h, 7 and 14 days after exposure, an increase in germ cell apoptosis was noticeable in early and late stages (I-VI and XI-XII) of the cycle of the seminiferous epithelium. However, apoptosis in intermediate stages (VII-X) was evident 16h after heat exposure (P