Recent work on the extraction and assay of urinary sex hormones is reviewed. The "sex hormones" known to be excreted in the urine are not identical with the hormones which have been actually isolated from the organs of secretion. Experimentally it is found that when active substances are administered only a very small proportion is excreted in recognizable form, and the excreted form may even be inactive. This general statement applies to oestrogens, androgens, progestin, and gonadotropic hormone. Hormone activity in the urine is, therefore, an uncertain index of hormonal activities in the body. However, with increasing knowledge of the chemistry and metabolism of hormones, methods of urine assay have been devised which give results capable of correlation with known or assumed physiological processes, notably in pregnancy and the normal menstrual cycle, and obvious effects are produced by certain types of tumour.In the case of male hormone activity the lack of relation between urinary androgens and sexual condition has led to the assumption that the androgens in urine are largely derived from sources other than the gonads, probably from the adrenal glands. Work on this question is described in detail.