The immediate response of the plasma prolactin level to oestrogen and progesterone infusions in bulls

The technique of radioimmunoassay was used to determine prolactin concentration in plasma taken from bulls during and after infusions of estradiol 17β and progesterone. During the infusion of estradiol 17β (0.5, 1 and 2 mg) for 1 hr the concentration of prolactin in the plasma fell, but showed an immediate increase of up to 6 fold after the infusion was completed. A dose dependent increase (up to 20 fold) in prolactin concentration was observed after the infusion for 1 hr of 5, 10, 40 and 80 mg progesterone. In those experiments in which 40 and 80 mg were administered the depression normally seen during the infusion was sustained for 2-3 hr and the subsequent increase during the next few hours was comparatively small.