Serum luteinizing hormone (LH) was measured throughout the afternoon and evening (1500-0130 h) of proestrus in young (4.5 month old) and middle-aged (10.5 month old) female rats displaying regular 4-day estrous cycles as judged by vaginal smears. An LH surge was measured in 22 of 24 young females. In 20 of the young females, the maximum concentration of LH in the blood was measured at 1800 h. The LH surge in middle-aged females was more variable in amplitude, and in 21 of the 29 animals in which a surge was observed it occurred at 1930 h or later. There was a relationship between the time of the surge and its amplitude; "late" surges were consistently lower. These observations demonstrate that marked changes in the pattern of LH secretion occur prior to the time ovarian cyclicity ceases in the aging female rat.