Pregnant CD-1 mice were treated with hexachlorobenzene (HCB) by gavage at doses of 0, 1, 10 and 50 mg HCB/kg body weight on days 6-17 of gestation and studied on day 1 or 21 postpartum (pp). Hearts of the dams and pups were assayed for lactic dehydrogenase (LDH) and creatine kinase (CK) activities and isozyme profiles. LDH and CK activities and CK isozyme profiles were not affected in the dams by HCB treatment, but isozyme LDH-5 was significantly depressed and isozyme LDH-3 significantly increased at the 50 mg/kg dose of HCB on day 1 pp. Hearts of the pups were studied on days 1, 8, 10, 15 and 21 pp. The 50 mg/kg dose level resulted in a statistically significant increased mortality in the pups. The mortality at the other doses showed a dose-related effect. LDH and CK activities and CK isozyme profiles were not affected by HCB exposure in utero. The only isozyme affected was LDH-5 with a statistically significant increase in the 50 mg/kg dose group on day 21 pp.