SUMMARY The prolactin concentration in the plasma of lactating rats rose less rapidly and attained a significantly lower plateau level in response to suckling on day 20\p=n-\21of lactation than it did on day 13\p=n-\14of lactation. Neither differences in suckling stimulation of the older pups nor a higher metabolic clearance rate (MCR) of prolactin were implicated in the reduced prolactin concentration seen in the late-lactating rats. The MCR was, in fact, slightly reduced in both conscious and late-lactating rats anaesthetized with urethane when compared with those in mid-lactation. The MCR of prolactin was not significantly altered by urethane anaesthesia in rats on either day of lactation. However, the secretion rate of prolactin, computed from the MCR multiplied by the equilibrum concentration of prolactin during suckling, was considerably reduced (665 to 392 ng/min) from mid-to late lactation. We conclude from these data that the reduced plasma concentration of prolactin in response to suckling in late lactation is the result of an impairment within the prolactin secretory mechanism
