The storage form of prolactin (PRL) was converted into the releasable form in the lactating rat pituitary gland within 10 min of suckling by 6 pups following 4–5 h of nonsuckling on postpartum day 13–14. The characteristics of the PRL discharge from the releasable pool into the circulation was then studied using a stimulus which is known to effectively release PRL (exposure of the mother to the exteroceptive stimuli emanating from 2 pups) but which is not of sufficient strength to influence the conversion of storage PRL. We found that the concentration of PRL could be repeatedly elevated to the same extent in the plasma with repetitive 10-min applications of this stimulus. With continuous 75 min of exposure to 2 pups, the plasma PRL concentration of the mother rose to a maximum within 15 min which then was sustained for the remaining 60 min, suggesting a steady release of PRL into the circulation had occurred. These data indicate that, unlike the storage form of PRL, the discharge of the releasable form occurs in relation to the length of time the stimulus is applied and exhibits neithe summation nor refractoriness. Subsequently it was noted that PRL could be released up to 8 h after the releasable pool has been formed and that the plasma concentration curves were not altered by injecting 5 mg of the lysosome inhibitor,. chloroquine. No elevation in plasma PRL concentration occurred when the pup stimulus was applied 16 h after formation of the releasable pool; chloroquine pretreatment, however, restored the normal elevation in plasma PRL levels in response to exposure to the pups, which suggests that releasable PRL may undergo enzymic destruction by pituitary gland lysosome after a critical length of time.