In a preliminary study, groups of ten female rats received daily doses of either water or solutions of Brown HT providing 250, 500 or 1000 mg Brown HT/kg body weight for 19 consecutive days. While there was no indication of overt toxicity, treated animals at all doses had brown discoloration of lymph nodes, caecum and colon. In the subsequent main study, groups of 30 female rats were given daily oral doses of 0 (water vehicle), 250, 500 or 1000 mg Brown HT/kg from day 0 to day 19 of pregnancy. There were no adverse effects on the numbers of implantations, pre- and post-implantation losses, litter weights, foetal numbers, foetal weights or sex ratio. No abnormalities related to treatment were found in either the skeleton or soft parts of the offspring. It is concluded that doses of up to 1000 mg Brown HT/kg/day given throughout pregnancy failed to exert detectable embryotoxicity or teratogenicity in rats.