Female Sprague-Dawley CD rats were fed 0, 60, 80, 100, 120 and 140 ppm hexachlorobenzene (HCB) continuously in the diet and 2 successive litters raised. These doses were selected to range from approximately the no observable effect level to lethality in suckling offspring of treated dams. In the F1a generation, the 21-day mortality was 9.2, 19.8, 30.0, 45.4, 93.1 and 92.6% in offspring of dams fed 0, 60, 80, 100, 120 and 140 ppm HCB, respectively. In the F1b generation, a similar mortality of 18.5, 21.5, 19.5, 45, 100 and 94.1% was observed at these 5 dose levels, respectively. The neonatal lethality observed was related to both maternal dose of HCB and the cumulative lactational exposure. Clinical signs of maternal toxicity were not observed and fertility and fecundity were unaffected. In the lungs of HCB treated dams, increased numbers of intraalveolar foamy histiocytes and hypertrophy and proliferation of the lining endothelial cells of pulmonary venules were observed. These microscopic findings of pulmonary effects of HCB confirmed previous findings of this laboratory.