PRL secretion in several physiological and experimental conditions, including early pregnancy, is linked to the daily photoperiod. The aim of this study was to examine the antepartum increase in PRL secretion for evidence of a circadian pattern of release, as seen during early pregnancy. During the last 3 days of pregnancy blood samples were taken six times daily by means of previously implanted jugular cannulae. Plasma PRL concentrations were then measured by RIA. PRL levels were less than 10 ng/ml in all animals on day 19 of pregnancy, but during the light period of day 20 there was an increase to an average of 30 +/- 10 ng/ml, with no evidence of a peak related to the time of day. However, in the dark period between days 20 and 21 there was a large surge of PRL secretion which reached peak levels of 356 +/- 39 ng/ml at 0500 h on day 21, then returned to 48 +/- 20 ng/ml at 1200 h, around the time of parturition. The peak always occurred at 0500 h and was not related to the time of parturition which ranged from 1000-2200 h on day 21. Bilateral ovariectomy (OVX) on day 19 advanced the time of delivery by approximately 12 h. In seven of nine animals, no surge of PRL secretion was observed during the dark period preceding parturition. Estradiol treatment after OVX on day 19 (OVX+E) advanced the time of delivery by approx 18 h. An antepartum PRL surge was present and was advanced by 24 h in all OVX+E animals, peaking at 0300 h on day 20. Progesterone treatment from day 18 to 21 in intact pregnant animals delayed parturition by approximately 18 h and prevented PRL secretion during the period of treatment. After progesterone treatment was stopped, a nocturnal surge of PRL secretion occurred, peaking at 0500 h on day 22, 24 h after the surge in normal animals. The results suggest that the increased PRL secretion during late pregnancy is linked to the daily photoperiod and is characterized by a nocturnal surge in the dark period preceding parturition. This surge is inhibited by progesterone, and it can be advanced 24 h by estradiol treatment in the absence of the ovaries.