Anterior pituitaries of normal adult male rats were subjected to synthetic thyrotropin-releasing hormone (TRH) treatment in an acute incubation system which employed pretreatment of the glands with plasma obtained from the donor animals. Following a 60-min preincubation period in a 1:1 mixture of Krebs-Ringer bicarbonate buffer (KRB) and plasma, media and hemipituitary prolactin (PRL) concentrations were significantly (p < 0.01) increased after a 40-min treatment with 500 pg TRH. The TRH effect was absent among hemipituitaries preincubated in KRB alone. Plasma obtained from older donors was more potent than was plasma from younger rats in this effect. TSH secretion was markedly increased by 500 pg TRH, whether or not plasma preincubation was employed. A dose response of PRL release to concentrations of TRH from 100 pg to 6.0 ng was observed. Crude extracts of median eminence also effected enhanced PRL release using the plasma preincubation technique. The results suggest that plasma preincubation of explanted pituitaries increases PRL cell sensitivity to TRH, perhaps by enzymatic inactivation of endogenous TRH bound to cellular membrane receptors.