Male and female Fischer-344 rats, CD (Sprague-Dawley origin) rats and B6C3F1 mice were exposed to 0, 5, 16, or 50 ppm nitrobenzene, 6 hours/day, 5 days/week for 90 days. There was no effect on body weight gain or mortality. Mean serum methemoglobin concentrations were significantly elevated in 5 ppm, 16 ppm and 50 ppm male F-344 rats; 16 ppm, and 50 ppm female F-344 rats; 16 ppm and 50 ppm male CD rats; 50 ppm CD female rats; 50 ppm male B6C3F1 mice; and 50 ppm female B6C3F1 mice. Elevations in methemoglobin concentrations were accompanied by hemotologic changes typically encountered with hemolytic anemia in the rat. Dose related changes secondary to methemoglobinemia and hemolytic anemia were seen in the spleens of all treated groups of rats but not mice. Interestingly, F-344 rats also had splenic capsular lesions similar to those described in the F-344 rats exposed to aniline while CD rats had only minimal splenic capsular lesions, and B6C3F1 mice had none. Both rat strains demonstrated sex differences in the occurrence of nephrosis, with males affected more than females. Cellular vacuolization of the zona reticularis of the adrenal was found in B6C3F1 female mice at the 5 ppm concentration, and increased in severity with dose. The liver was affected in F-344 rats, CD rats, and B6C3F1 mice.