The effect of exercise on the production of ovarian progesterone was examined in female rats. During in vivo experiments, diestrous rats were catheterized via the right jugular vein (RJV), and blood samples were collected before and after 10, 15, 30, and 60 min of swimming. In addition, blood samples were collected from the RJV before and 2, 5, 10, 15, 30, 60, and 120 min after 10 min of infusion of lactate (13 mg.kg-1.min-1) through the left femoral vein. To explore if lactate modulates progesterone secretion by acting directly on rat ovary or on anterior pituitary gland (AP), an in vitro experiment that mimicked the in vivo condition was performed. The ovarian tissue was challenged with lactate (0.01-10 mM) or porcine follicle-stimulating hormone (1 microgram/ml) and 3-isobutyl-1-methylxanthine (1 mM) for 60 min, and the AP was challenged with lactate ranging from 0.1 to 10 mM or 10 nM gonadotropin-releasing hormone for 30 min. The postexercise levels of plasma glucose, lactate, and progesterone at 10, 15, and 30 min were significantly higher than the corresponding basal levels. Plasma luteinizing hormone (LH) did not change after exercise. An elevation of plasma lactate and progesterone was found at 15 and 30 min subsequent to 10 min of infusion of lactate. Lactate ranging from 0.01 to 10 mM significantly increased ovarian adenosine 3',5'-cyclic monophosphate (cAMP) and progesterone production in a dose-dependent manner. LH concentration in plasma was not changed subsequent to lactate infusion. LH level in media samples was not altered after incubation of AP with lactate. These results suggest that the increase of plasma progesterone level in rats during exercise is independent of LH secretion and at least in part is due directly to a stimulatory effect of lactate on the production of ovarian cAMP.