The ovary and its vascular pedicle were isolated and transferred under a stereomicroscope to a heated surface (40 degrees C). The ovary was supplied with the blood from the middle uterine artery through a cannula. On different days of the oestrous cycle [3H]testosterone, [3H]progesterone or [3H]oestradiol-17 beta were infused for 30 min into the ovarian vein 1--2 cm below the ovary. During and 30 min after the infusion radioactivity was found in ovarian arterial blood and ovarian tissue. Recovery of [3H]testosterone, [3H]oestradiol and [3H]progesterone infused into the ovarian vein from ovarian venous blood collected during 60 min of experiment was (mean +/- s.e.m.) 26.8 +/- 6.8%, 20.1 +/- 6.1% and 23.7 +/- 6.2% respectively. When 51Cr-labelled blood blood cells were infused in the same fashion as the steroids there was no radioactivity in arterial blood or ovarian tissue. These results indicate the existence of a countercurrent transfer mechanism and retention of the hormones in tissue(s) other than blood in the ovarian pedicle.