The effects of suckling and LH upon serum progesterone levels in lactating hamsters were investigated. Diurnal rhythms in serum levels of progesterone and LH occur throughout lactation in the hamster and these levels are normally higher between 15.00 and 17.00 than between 8.00 and 12.00 (lights on 6.00-20.00). Hamsters injected with 10 mug of LH at 8.00 on day 9 of lactation had higher levels of serum progesterone 2 h after injections than did vehicle-injected controls. Suckling was also found to induce an increase in serum progesterone concentrations. 30 min of suckling from 10.30 to 11.00 on days 10 or 11 of lactation resulted in increased serum levels of progesterone and prolactin, but had no measurable effect upon serum LH levels in animals bled at 11.00. However, suckling for 30 min from 15.00 to 15.30 had no effect upon serum progesterone levels, although it did result in increased titers of both serum LH and prolactin at 15.30. Ovariectomy on day 7 of lactation resulted in lowered levels of serum progesterone at 11.00 on day 11 of lactation and abolished the suckling-induced increase in serum progesterone concentrations. These findings suggest that in the lactating hamster (1) suckling bouts stimulate progesterone secretion from the ovaries, and (2) the diurnal rise in serum progesterone levels is stimulated in part by the increase in serum levels of LH.