Polyacrylamide disc gel electrophoresis has been recently introduced for the assay of rat anterior pituitary gland hormones, particularly of prolactin and of growth hormone (GH). The most important step in this method, on which its reliability and quantitative yield depend, is the extraction of hormones from the pituitary gland. This paper reports a comparison between different methods of extraction already described in the literature, and our own procedure, which seems to give a quantitative extraction of the GH and the prolactin present in the pituitaries. For the extraction of the hormones, female rat anterior pituitaries were homogenized (3%, w/v) in 0·02 m-phosphate buffer, pH 7·0, according to Kurcz, Nagy & Baranyai (1969); in water, according to Yanai & Nagasawa (1969); in acrylamide sample gel, pH 8·6, according to Nicoll, Parsons, Fiorindo & Nichols (1969); or in 0·32 m-sucrose. This last homogenate was diluted 1:1 with 0·02 m-phosphate