Attention has been directed recently toward human urine as a source of supply of several hormonal substances. Aschheim and Zondek1reported constant and characteristic responses in the ovary of the immature mouse to injection of the whole urine of pregnancy which they have ascribed to a hormone of the anterior lobe. Louria and Rosenzweig2have reported similar results. It is not to be denied that the urine from pregnant women elicits an ovarian response similar in some respects to that induced by implants of the fresh anterior lobe. Other features of this response, however, differ greatly from those produced by the fresh gland. It is to these atypical features which appear not to have been appreciated by recent workers in this field, and which make untenable their assumption that this is a typical anterior pituitary response, that attention is called here. In the present experiments, immature mice, from