The time-effect relationship of the behavioral and hematological changes caused by inhaled benzene was investigated in C57BL mice. Mice were exposed to air, or 100, 300, 1000, or 3000 ppm benzene in standard inhalation chambers employing dynamic vapor exposure techniques. Mice were exposed for 6 hr/day for the number of days necessary to achieve a minimum concentration X time (Ct) product of 3000 ppm-days. The intermittent exposure regimen of 6 hr/day was employed to simulate occupational exposure. The most sensitive behavioral index (milk-licking) was affected by the lowest concentration tested (100 ppm), while homecage food intake, hindlimb grip strength, and body weight were reduced only at 1000 and 3000 ppm. Some of these previously undocumented behavioral changes occurred as rapidly as hematological changes that have been considered hallmarks of benzene toxicity. A significant decrease in circulating lymphocytes occurred after exposure to all concentrations. Circulating red blood cells were variably affected by benzene, in that anemia resulted after 10 days exposure to 100 ppm and after 3 days exposure to 300 ppm but not after 3 days exposure to 1000 ppm or a single exposure of 3000 ppm. The data indicate a departure from Ct relationships, and suggest that exposure duration as well as daily dose may be an important factor in benzene toxicity.