The identity of the pituitary factor responsible for the maintenance of a female pattern of hepatic steroid metabolism and a female level of PRL receptors has been established. Fractionation of pituitary extracts revealed that only the GH fraction had the capacity to feminize liver metabolism of androstenedione (i.e. increase 5 alpha-reductase activity and decrease 16 alpha-hydroxylase activity) and to induce PRL receptors to a female level in hypophysectomized animals. The purification of pituitary GH was performed by chromatofocusing followed by gel filtration on Sephadex G-75. GH obtained from male or female pituitary glands showed an identical chromatographic behavior and both preparations had a mol wt of 22,000 and an isoelectric point of 6.1 when analyzed by analytical sodium dodecyl sulfate-gel electrophoresis and isoelectric focusing, respectively. The degree of homogeneity of GH varied between 93% and 97% as judged from sodium dodecyl sulfate-gel electrophoresis. Purified male and female GH were equally efficient in feminizing the liver metabolism. Since degradation of the native mol wt 22,000 form reduced the feminizing capacity, we believe that the intact hormone is needed for the feminization.