(fig. 7). 3. The cyclic changes in the milk ejection parameters caused by PGF 2 Πwere closely related to the plasma progesterone level (figs. 2, 3, 5, 6). The coefficients of correlation between the progesterone level and the -threshold dose : r = -0.809 ** (group AI, -volume of ejected milk : r = + 0.872 *** (group B), -deflection amplitude : r = + 0.805*!&dquo;!&dquo; (group B) were always significantly higher than the threshold of 0.001.Various hypotheses concerning the endocrine control of cyclic variations in milk letdown under the effect of PGF 2 Πare discussed.Introduction.