To study the function of the corpus luteum (CL) through its major secretory product, progesterone (P4), catheters were inserted into the carotid artery (via the facial artery) and the ovarian vein (n = 12), and electromagnetic flow transducers were placed around the ovarian artery in cycling Angus and Hereford cows (n = 6). Blood samples were taken four times daily (at 0600, 1200, 1800 and 2400 h) and ovarian blood flow (OBF) was monitored for 60 min immediately after each blood sampling. After chromatography, P4 was measured by radioimmunoassay. The P4 concentrations in the ovarian vein (OP4) were correlated with day of the estrous cycle (r = .25; P less than .05) and were higher during the morning hours (P less than .05). Arterial progesterone (SP4) was correlated to OP4 (r = .24; P less than .05) and day of the cycle (r = .35; P less than .05). The OBF changed among days (P less than .05). The highest rates were noted during luteal maturation (23 ml/min; SE = .09) and the lowest were noted with the demise of the CL (SP4 less than 1 ng/ml) and approach of estrus (8 ml/min; SE = .07). The OBF was correlated with SP4 (r = .24; P less than .05), although no within-day trends were noted. Exogenous estrone (6 mg) administered via jugular vein decreased OBF within 30 to 45 min, but similar injections of P4 (up to 100 mg) had no effect. Progesterone release (P4R) from the ovary [(OP4 - SP4) x OBF] was higher in the morning hours (P less than .05). The P4 concentration (OP4, SP4) and release (P4R) exhibited wide variations among and within days. The changes in OP4 and P4R were both good indicators of CL development, maturation and regression, as associated with SP4 changes. Oxygen (O2) and carbon dioxide (CO2) concentrations monitored in the carotid artery and ovarian vein indicated that the ovary with the CL was not limited in O2 availability or CO2 removal during periods of low blood flow or high secretion of P4.