We have previously demonstrated that mild testicular hyperthermia induces stage-specific and germ cell-specific apoptosis in rat and mouse testes. The objectives of this pilot study were to examine whether mild testicular hyperthermia induces azoospermia and oligozoospermia in nonhuman primates, and to determine whether spermatogenesis suppression was due to acceleration of germ cell apoptosis. Three adult Cynomolgus monkeys (Macaca fascicularis) were used in this study. The scrota containing the testes were immersed in a water bath at 43 degrees C for 30 minutes once daily for 6 consecutive days. Semen and blood samples were collected at 2 and 1 weeks before, and 2, 4, 6, 8, 10, and 12 weeks after the first heat treatment. Testicular biopsies were performed before and at 3 and 7 days, and 12 weeks after the first heat exposure. Apoptosis in testicular biopsy was assessed by TUNEL assay, by electron microscopy, and by detection of cleaved Poly(ADP-ribose)polymerase with Western blotting. A transient decrease in serum testosterone levels was observed in 2 monkeys 2 weeks after heat treatment. Serum inhibin B levels declined in all 3 monkeys 2 weeks after testicular hyperthermia and remained at relatively low levels throughout the study in 2 of 3 monkeys. Two of 3 monkeys exhibited azoospermia by 6 or 8 weeks after the first heat treatment; the remaining monkey had marked oligozoospermia (8 x 10(6)/ejaculate, 10.89% of pretreatment levels) 6 weeks after the first heat treatment. Increased germ cell apoptosis in testicular biopsy samples was found at 3 and 7 days after the first heat exposure. Using immunohistochemistry, we observed that the immunoactivity of proapoptotic Bax protein accumulated in heat-induced apoptotic germ cells. Full recovery of spermatogenesis was noted 12 weeks after the first heat treatment. We conclude that, similar to rodents, mild testicular hyperthermia results in azoospermia and oligozoospermia in monkeys through increased germ cell apoptosis with minimal effect on the hormonal milieu.