Weanling male Sprague-Dawley rats were gavaged 5 d/wk with 1,3-dinitrobenzene (m-DNB) at dosages of 0, 0.75, 1.5, 3.0, and 6.0 mg/kg X d. Males were bred to untreated females during treatment wk 10 and were killed during treatment wk 12. Although males dosed with 3 mg/kg X d inseminated the females and evidence of mating was observed in males dosed with 6 mg/kg X d, none of the males in these groups sired litters. Diminished sperm production (reduced testicular sperm head counts), decreased cauda epididymal sperm reserves, nonmotile spermatozoa, atypical sperm morphology, decreased weights of the testes and epididymides, seminiferous tubular atrophy, and incomplete spermatogenesis were also observed in these groups. Sperm production was also decreased in males dosed with 1.5 mg/kg X d. Changes in the spleen included increased weight at dosages of 1.5 mg/kg X d or higher and splenic hemosiderosis, which ranged from slight in rats treated with 0.75 mg/kg X d to moderately severe in those dosed with 6 mg/kg X d. The data indicate that m-DNB is a potent testicular toxicant in the male rat, capable of producing extensive damage to reproductive tissues and reproductive failure. Limited data on four rats that received 6 mg/kg X d and were allowed a 5-mo posttreatment recovery period suggested that the testicular effects are at least partially reversible.