Twenty-eight crossbred gilts were assigned randomly in equal numbers to four subgroups in an experiment of 22 factorial design. All gilts were individually fed a basal grain ration each day. Treatment consisted of feeding gilts 125 mg of methallibure (1-α-methy-lallylthiocarbamoyl-2-methylthiocarbamoylhy-drazine) daily, mixed with ration, beginning on the first day of detected estrus. Autopsy was performed on day 8 or 16 of the estrous cycle (first day of estrus=day 0 of the cycle). Treatment caused an increase in pituitary FSH and LH activity at each stage of the cycle studied (FSH, P<.01; LH, P≅.07). Levels of pituitary FSH and LH activity were significantly greater at day 16 than day 8 of the estrous cycle. The stalk median eminence was heavier and contained significantly more LRF at day 16 than day 8 of the cycle. Feeding methallibure to gilts failed to affect the weight or luteinizing hormone releasing factor (LRF) activity of the stalk median eminence. These data suggest that methallibure does not alter levels of pituitary LH by interfering with LRF activity. Corpora lutea in both control and treated gilts weighed significantly more at day 8 than day 16 of the estrous cycle. Treatment tended to reduce the weight of corpora lutea at day 8 and increase the weight of these glands at day 16 but the differences were not significant statistically. Methallibure caused a reduction in luteal progesterone content and concentration at day 8 but an increase in luteal progesterone content and concentration at day 16 (day × methallibure interaction, P<.01, for each characteristic). In both control and treated gilts ovarian follicular fluid was significantly greater at day 8 than day 16 but less in treated than control gilts at each stage of the cycle studied (P<.01). Treatment reduced the number of ovarian follicles @?@ mm in diameter, particularly at day 16 of the cycle.