Tubule segments from defined stages of the mouse seminiferous epithelium were incubated for 4, 8 and 24 h in the presence of different concentrations of mono-2-ethylhexyl phthalate (MEHP), adriamycin, and N-ethyl-N-nitrosourea (ENU) to study the effects of these chemicals on germ cell apoptosis and DNA synthesis. Apoptosis was measured by using the in situ end-labeling (ISEL) technique and microscopic analysis of apoptotic cells, and DNA synthesis was assessed by 3H-thymidine incorporation. In the mouse testis after 8 h incubation, MEHP increased the number of apoptotic cells significantly at 0.01 and 0.1 mM concentrations in stages IX-XI of the mouse seminiferous cycle. Adriamycin increased the number of apoptotic cells significantly at 0.1 and 1.0 microM concentrations in stages IX-XI. In tubules from stages IX-XI cultured in the presence on 0.01 and 0.1 mM ENU the number of apoptotic cells was increased significantly. In dose-response experiments trend of decreasing amount of DNA synthesis was observed, but no statistical differences were found. MEHP, adriamycin and ENU increased the amount of apoptotic cells in model where transillumination assisted microdissection was combined to ISEL. We conclude that ISEL technique provides a useful method to study the stage-specific effects of chemicals on spermatogenesis.