Postparturient rats were ovariectomized on day 1 postpartum and allowed to nurse either 2 or 9 pups. On days 6 or 11, the 9–pup litters were discarded and the 2–pup litters were increased to 9 pups. The mothers' serum luteinizing hormone (LH) levels were determined by radioimmunoassay immediately before, and at intervals up to 7 days after the change in litter size. Controls were: ovariectomized on day 1, with litters of either 2 or 9 pups or with pups removed; sham ovariectomized on day 1, with litters of either 2 or 9 pups. In all control groups, litter size was not changed but pups were replaced with pups of the same age at the same intervals after delivery as in the experimental groups. In the intact control rats and in the ovariectomized controls nursing 9 pups, serum LH was well below 2.0 ng equivalent of LH—LER–1240/ml until day 11, and rose only slightly on or after day 12. In the ovariectomized controls with 0– or 2–pup litters, however, the serum LH levels were between 3.6 (2 pups) to 5.0 (0–pups) ng/ml on day 6, and reached approximately 20 ng/ml by day 18. Removal of the 9–pup litters on either days 6 or 11 promptly increased serum LH; within a week the values reached those of the 0–pup controls. Increasing the litter size from 2 to 9 pups on day 6 reduced serum LH but not to the level of the 9–pup controls; the level tended to rise again between the 4th and 7th day after the increase in litter size. The same changes occurred when litter size was increased on day 11, but with a less pronounced fall and a more marked tendency to rise. These findings were discussed in relation to the opposing effects of ovariectomy and suckling on the dynamics of LH secretion. (Endocrinology92: 206, 1973)