Twenty one New Zealand White rabbits had both ovarian veins cannulated under nembutal anesthesia, and blood was collected for 30 mm. Twelve rabbits were in estrus, six had received an injection of HCG between 2–6.5 h, and three had been mated 2–3 h before blood collection. The concentrations of estrone (E1), estradiol (E2) and progesterone (P) were measured by radioimmunoassay following separation on celite microcolumns. Substantial differences in concentrations of E1, E2 and P between the two ovarian veins of individual estrous animals were observed in five of nine, six of 12, and two of 12 rabbits, respectively. The ratios of the steroid concentrations between the two ovarian veins ranged from 1.0–2.1 for E1, from 1.0–25.5 for E2 and from 1.1–64.1 for P. After mating or HCG injection substantial differences between ovaries for E1, E2 and P were seen in three of six, two of nine, and eight of nine animals, respectively. Compared to the estrous animals, more rabbits showed such differences only for P and fewer rabbits differences for the other steroids. E1 and E2 levels did not rise after mating or HCG injection, but by contrast P values rose significantly. A second injection of HCG given 6.5 or 7 h after an initial one caused, within the succeeding 45 min, a further stimulation of total progestin concentration and some individual stimulation of total estrogen concentrations. These results provide clear evidence for differences in steroid secretion between the left and right ovaries of the same animals.