Benomyl, a systemic fungicide, was administered to male Sprague-Dawley rats during the prepuberal, pubertal, or postpubertal stage of reproductive development. Animals received 5 or 10 daily treatments of 0, 125, 200, 250, 500, or 1000 mg benomyl/kg . d by gavage. Observations were made at selected intervals after exposure and included hematological parameters, body weight, tissue weights, total epididymal sperm counts, vas deferens sperm concentration, serum follicle-stimulating hormone ( sFSH ) levels, and testicular histology. Data presented here suggest that there is an age-related difference in sensitivity to benomyl. Animals that received benomyl treatments during prepuberty showed no significant treatment effects in tissue weights, total epididymal sperm counts, vas deferens sperm concentration, or sFSH . Animals that received at least 250 mg/kg . d during puberty or postpuberty showed one or more of the following effects: decreased testicular or epididymal weights, decreased epididymal sperm count, decreased vas deferens sperm concentrations, and/or testicular lesions. Histological examination of testicular tissue indicated a higher incidence of diffuse hypospermatocytogenesis in pubertal (20% of the treated animals) and postpubertal (40% of the treated animals) animals that were exposed to benomyl. These values were compared with those of the treated prepubertal animals, which had a 10% incidence of diffuse hypospermatocytogenesis , and with all of the control animals, which had no occurrences of this testicular lesion.