Hexachlorobenzene (HCB) is a known reproductive toxin. However, the full spectrum of its reproductive toxicity is unknown. Consequently, the effect of HCB on serum oestradiol (E2) and progesterone (P4) concentrations during the follicular (days 1-9), periovulatory (days 10-14) and luteal (days 15 to beginning of next menses) phases was investigated in the spontaneously cycling cynomolgus monkey. Adult female cynomolgus monkeys (n = 16) were randomly assigned to one of four treatment groups and orally doses with gelatin capsules containing HCB (0.0, 0.1, 1.0 and 10.0 mg kg-1 body wt. day-1) mixed with glucose. A 10-week acclimitization phase was followed by 13 weeks of dosing. HCB induced a dose-dependent suppression of serum P4 concentrations during the luteal phase. However, circulating levels of P4 were unaffected during the follicular and periovulatory phases of the menstrual cycle. Serum E2 concentrations, body weight, menstrual cycle length and duration of menses were not affected by HCB treatment. The range of menstrual cycle length and duration range of menses, however, were broader in the highest dose group. We conclude that HCB interfers with mechanisms regulating ovarian steroidogenesis and suppresses P4 levels during the luteal phase in the cynomolgus monkey.