Rat anterior pituitaries were incubated in vitro 10 min with a 3H-amino acid mixture and then transferred to chase medium without label for 1, 3, and 6 hr. During the first hour of chase there was a much slowed but continuing incorporation of label into total protein, prolactin (PRL) and growth hormone (GH). However, if puromycin was added to the chase medium, there was a drop in the level of all labeled protein species during the first hour of chase. Total label (pituitary plus medium) in all labeled protein species then stabilized during the remaining 5 hr of chase. During the 6-hr chase in non-puromycin medium there was an apparent greater medium gain of labeled total protein and PRL than was lost from the pituitary. Conversely, in puromycin chase medium there was a greater 6-hr loss of pituitary cpm than could be accounted for by medium gain. However, if medium gains were compared to pituitary losses only during the last 5 hr of chase, then the two were equivalent for both puromycin and non-puromycin medium. Labeled total protein and PRL release were slowed in puromycin as compared to non-puromycin medium; conversely, the rate of labeled GH release was greater. When dibutyryl cyclic AMP was added to the puromycin chase medium there was a significant stimulation of labeled total protein, PRL and GH release. With a colchicine plus puromycin medium, release of labeled total protein and GH was enhanced; after 6 hr of chase labeled PRL release was marginally inhibited. In a Ca++-free puromycin medium, release of labeled total protein and PRL, but not of GH, was inhibited. (Endocrinology93: 767, 1973)