The study determined the quantitative and qualitative histopathologic effects of a single oral dose of 1,3-dinitrobenzene (48 mg/kg) on the rat testis from 1 to 175 days postexposure. The testis was damaged severely by hour 24, as evidenced by increased numbers of regressive seminiferous tubules that exhibited degenerating pachytene spermatocytes, chromatin margination in spermatids, and reduced numbers of meiotic figures. The major effects during the first 48 hours posttreatment were degeneration or exfoliation of pachytene spermatocytes and round spermatids and the retention of step 19 spermatids. These regressive effects continued until 24 days, after which the tubules either recovered or became atrophic. At the end of the study (175 days), three males were normal, one had regressed testicles, and three males had atrophic tubules (15 to 45%). Several cellular abnormalities were common throughout the period.