Hexachlorobenzene (HCB) residue levels in dosed rats (50.0 mg kg-1 body wt.day-1, n = 9) were significantly (P < 0.05) greater in the periovarian fat compared to the thyroid gland. Hexachlorobenzene residue levels were significantly (P < 0.05) greater in the thyroid versus the adrenal and ovary. Ovarian HCB residue levels were greater than those found in the thymus, liver and lung. Serum thyroxin (T4) and the free T4 index (FTI) were significantly (P < 0.05) suppressed in HCB-treated rats compared to the control group (n = 8). In contrast, no significant differences in serum concentrations of oestradiol (E2), progesterone (P4) or percentage triiodothyronine uptake (%T3) were observed, thus suggesting an HCB-induced hypothyroid-like state. In a second experiment, adult female Sprague Dawley rats (n = 16) were dosed as above and superovulated with pregnant mare serum gonadotrophin (PMSG, 10 IU s.c.) and human chorionic gonadotrophin (hCG, 20 IU s.c.). Circulating levels of P4 were significantly (P < 0.05) elevated compared to the control group (n = 8). The %T3 uptake and serum T4 levels were significantly (P = 0.05) suppressed compared to controls. Hexachlorobenzene treatment had no effect on circulating levels of E2 or on the FTI. These results suggest that HCB-induced changes found in the spontaneously cycling rat are augmented by ovulation induction strategies. We also conclude that HCB concentrates in the endocrine tissues in addition to the fat.