The purpose of the study reported here was to define strain differences in spermatogenesis in cryptorchid testes in mice. Mice of strains A/J, BALB/c, CBA/N, C3H/He, C57BL/6 (B6), ddY and ICR were found to be sensitive to heat stress attributable to experimentally induced cryptorchidism. In contrast, mice of strains AKR/N (AKR), MRL/MpJ-+/+ (M+) and MRL/MpJ-lpr/lpr (lpr) were resistant to heat stress. Relative increases of apoptotic cells were detected in the sensitive group, but not in the resistant group. A decrease of proliferating cell nuclear antigen-immunoreactive cells after experimentally induced cryptorchidism was observed only in the sensitive group. These results suggested that heat stress-resistant germ cells were present in MRL and AKR strains, possibly originating from the genetic background.