Lead is a male reproductive toxicant. We previously reported that in vivo lead exposure results in a suppression of the hypothalamic-pituitary- testicular axis in male rats. This study was designed to assess if lead exposure in vivo alters (1) the ability of sperm to fertilize ova in vitro, (2) the morphology of the spermatozoa, and (3) the relationship of cell types in the testes as evaluated by DNA flow cytometry. Male Sprague-Dawley rats were given access to either lead-free (0.0%) or 0.3% lead acetate-containing water for 14, 30, or 60 days starting at 100 days of age. On the day of termination, sperm harvested from the caudae epididymis were incubated with eggs harvested from superovulated non-lead-treated females and were scored for stages of penetration. Sperm were also studied by electron microscopy. The testes of animals treated for 60 days were processed for DNA flow cytometry. The overall distribution of the stages of fertilization was significantly different between control and lead-treated animals. The lead- treated groups fertilized significantly fewer eggs than did sperm from the control group. Increased duration of exposure to lead acetate did not result in a more significant percentage of eggs not fertilized. No ultrastructural changes were noted in the spermatozoa of animals treated with lead compared to control animals. There were no differences in the histogram patterns of testicular cells collected from lead-treated animals and control animals. We conclude that lead alters sperm function by altering the hormonal control of spermatogenesis rather than by direct toxic action on spermatozoa.