it is possible that pronase enhances the ability of the pituitary to respond to secretory stimuli.The mechanism of this enhancement is not clear, and the biochemistry of the events which intervene between the addition of prostaglandin E2 or increase in the K + concentration and the release of growth hormone is equally not known. Prostaglandin E2 may act by increasing the concentration of cyclic AMP in the pituitary (2), and high K § leads to depolarization (6) and increased Ca 2+ incorporation (11) without changing the cyclic AMP concentration (10). Since pronase potentiates the effect of both stimuli it is probable that it affects a fundamental property of the secretory system, for example, membrane Ca 2+ permeability.Alternatively, since the release of growth hormone is known to be inhibited by the hypothalamic peptide somatostatin (i), pronase could increase release by destruction of somatostatin present in the tissue at the time it was removed from the animal.We thank Mrs M. Sidler-Ansermet and Miss M. Ravazzola for help.