Three different aspects of the control of prolactin secretion in birds are discussed. 1. (1) The problem of autonomy of prolactin secretion in birds. Four bird species have been examined in this respect. The analysis of results obtained from long-term culture experiments and from cytological studies on cultured or autografted adenohypophysis leads to the conclusion that there is autonomous prolactin secretion in the Pekin duck, but not in the pigeon, and probably not in the tricolored blackbird and the quail. 2. (2) The in vitro evidence and modalities of action of the prolactin-stimulating factor(PSF). The existence of a prolactin-stimulating factor in bird hypothalamic extracts appears as a general fact consistently observed in the 6 species examined. By long-term experiments it was shown that PSF stimulates both synthesis and release of prolactin and that increased KCl concentration does not exert the same degree of stimulation. In short-term incubation experiments, it appears that the effects of the PSF depend on the initial prolactin content of the pituitary. If the latter is high, the PSF acts more on release, but if it is low, the most conspicuous effect observed was on synthesis. The PSF content of the hypothalamus does not vary to the same extent as the pituitary prolactin level in the duck, quail, and turkey, but the contrary was observed in the pigeon. 3. (3) The evidence for additional hormonal control of prolactin cells. In the Pekin duck and the quail, testosterone (0.5 μg/ml) inhibits prolactin release and induces appearance of lysosomes which could participate in the intracellular degradation of secretory product.