An aqueous solution of trioxane was administered by gavage to female rats, 5 days per week for 7 weeks, at doses of 0.19, 0.58 and 1.16 g/kg/day. A significant increase in the mean duration of the oestrous cycle, mainly due to lengthening of the dioestrus, was only noted in the 6th and 7th week of treatment in females given per os trioxane at a dose of 1.16 g/kg. Three weeks after cessation of treatment, no alterations of oestrous cycle were observed in all groups. Since a dose-related decrease in body weight gain was observed in females given trioxane by gavage at doses of 0.19-1.16 g/kg and trioxane-induced behavioral changes were seen in the 1.16 g/kg group, it was concluded that exposure to trioxane did not affect the sexual cycle unless other overt signs of trioxane toxicity were induced.