The dependence of spermatogenesis on the relatively cool environment of the scrotum is well known, and recent work has shown that germ cells undergo apoptosis upon exposure to abdominal temperature. p53 is a potent inducer of apoptosis and regulator of cell growth, and is found in high concentrations in the testis. The purpose of this study was to determine whether exposure of the testes to suprascrotal temperature was associated with alterations in testicular p53 expression. Male adult CD-1 mice were rendered unilaterally cryptorchid by surgically fixing one testis to the anterior abdominal wall while leaving the contralateral tests in the scrotum to serve as the euthermic control. p53 expression was evaluated in the cytoplasmic, soluble nuclear, and insoluble fractions by Western blot analysis with the monoclonal p53 antibody pAb240. The weights of the scrotal testes were unchanged over the 15 day study period. The weights of the cryptorchid testes remained stable for 7 days and then decreased by approximately 40% over the next two days. Histological evidence of germ cell loss was evident only after day 7. Altered expression of p53 protein in the cryptorchid testis was noted beginning on day 7, and consisted of the expression of a new 47 kD isoform of p53 in the cytosolic form and a 30 kD isoform in the soluble nuclear fraction. Scrotal testes showed no changes at any time point. These results demonstrate altered expression of the regulatory protein p53 beginning 1-2 days prior to the onset of germ cell loss following experimental unilateral cryptorchidism. Given the known function of p53 as an inducer of apoptotic cell death, these observations suggest a significant role for p53 in temperature-mediated germ cell loss.