The temporal relationship between changes in mammary blood flow (MBF) and changes in the concentration of plasma prolactin, progesterone, estradiol-17beta, and cortisol, was examined in chronic sheep preparations undergoing spontaneous labor (Group I) or labor induced by an infusion of dexamethasone (1 mg/24 h) to the fetus (Group II). In Group I, an increase in prolactin (45 to 489 ng/ml), a decrease in progesterone (15 to 4 ng/ml), and an increase in MBF (97 to 365 ml/min) occurred at about the same time, whereas increases in estradiol-17beta (80 to 211 pg/ml) and cortisol (9 to 39 ng/ml) followed the change in MBF. A similar pattern of changes in MBF and hormonal concentrations occurred over a shorter period when premature labor was induced in the animals in Group II. These findings suggest that changes in plasma prolactin and progesterone concentrations play an important role in the regulation of MBF at the time of parturition.