Summary Plasma LH was measured by the one-hour OAA depletion assay in cyclic, ovariectomized and hypophysectomized rats. Plasma collected between 2:30 and 3:50 PM of proestrus contained as much circulating LH as is seen following ovariectomy; not as much LH was in plasma at the same time on the afternoon of estrus. The 5-day cyclic rats had significantly more LH in plasma at both proestrus and estrus than was found in 4-day cyclic rats. Pentobarbital administered at 2 PM (at a dose level which blocks ovulation in the proestrous 4-day rat) reduced plasma OAA depleting activity to the minimal amount found in hypophysectomized rats.