Diets containing the herbicide dinoseb were fed to adult male Sherman rats for 11 weeks. One-half the survivors were killed for terminal studies during the 11th week and the remainder bred to untreated females during posttreatment, and then killed for terminal studies. Interim sacrifices were made in groups fed 0 and 300 ppm. In rats fed 300 ppm, differential classification of spermatozoa from the cauda epididymis indicated 90% of the cells were atypical by 20 days of treatment. By 30 days, bizarre and amorphous forms were observed and epidiymal sperm counts were decreased. Histologic changes in the testes included abnormal spermatozoa and spermatids and multinucleated spermatogenic cells at 20 and 30 days and severe damage to spermatogenic cells by 50 days.