The hybrid transgene approach was adapted to study the physiological pathway(s) in which the p53 suppressor gene is involved. p53 promoter-CAT transgenic mice were found to express enzymatic CAT activity predominantly in the testes. In situ hybridization indicated that expression of the transgene as well as the endogenous p53 agreed with the typical wave and cycle patterns of spermatogenesis. p53 promoter-CAT transgenic mice expressed in the testes reduced levels of endogenous p53 mRNA that correlated with the copy number of the mouse or human transgene. The spatial and cyclical expression of the p53 gene which is confined to the primary spermatocytes in the seminiferous tubuli suggested that p53 may play a role in the meiotic process of spermatogenesis in vivo.