Prolactin in blood plasma samples obtained from cows was determined by radioimmunoassay. During infusion of 2 mg or 4 mg 17β-oestradiol/hour there was a depression of the prolactin level, which seemed to be pronounced during infusions of 6 or 12 mg 17β-oestradiol/3 hours. In the experiments in which 2-6 mg 17β-oestradiol were administered a 2-5 fold increase in the plasma prolactin concentrations occurred after the infusions were completed. In the experiment with 12 mg 17β-oestradiol the depression was sustained for 3 hours after infusion; the subsequent increase the next few hours was comparatively small.