Many studies have suggested that rats exhibiting 4-dayestrous cycles may differ from those exhibiting 5-day estrous cycles in hormonal secretion rates. The present study was carried out to delineate the pattern of the ovulatory surge of LH secreted in these two types of cycles. During the day of proestrus, 1–3 serum samples per rat were collected by cardiac puncture, under ether anesthesia, at 30-min intervals, from 13.30 to 20.30 h. Serum LH was determined using the 0:0 RIA. No rat had values above 10 ng/ml before 14.30 h and individual animals showed a high rate of LH release for variable lengths of time. Serum LH values did not differ significantly between 4- and 5-day cycles over the times observed. At 13.30, pentobarbital, which has been reported to be less effective in blocking ovulation in the 5-day cyclic rat, lowered serum LH in both 4- and 5-day groups to 1–17 ng/ml over an identical time period. However, at 15.30 h the 4-day pentobarbital-treated animals had a significantly higher LH level. This may have been due to an effect of cardiac puncture at 15.00 h.