Hexachlorobenzene (HCB) exposure has been shown to alter the normal concentrations of parathyroid hormone and 1,25-dihydroxy vitamin D3 in rats and to result in osteoporosis in humans. Experiments were undertaken to investigate the effects of HCB on the homeostatic mechanism of calcium metabolism and to determine its effect on bone in rats. Fischer 344 rats were dosed 5 days/wk for 5, 10 or 15 wks with 0, 0.1, 1.0, 10.0 or 25.0 mg HCB/kg body weight. Body weight was not affected by any of the exposure conditions. Liver weight was significantly elevated above control values at the two higher dose levels at all three time periods. Kidney weight and kidney-to-body-weight ratio was significantly elevated at the highest dose level after 10 weeks and at the two higher dose levels after 15 weeks of exposure. Serum alkaline phosphatase was significantly decreased at the two higher dose levels after both 10 and 15 weeks of exposure. 1,25-Dihydroxy vitamin D(3) was measured in the 5 week exposure group only and was significantly elevated in the three higher dose levels. After 5 and 15 weeks of exposure, parathyroid hormone concentration was significantly elevated at the two higher dose levels at both time periods.