Organ cultures of sexually differentiated gonads were studied in order to determine the role played by the gonadotropic hormones LH and FSH in the development of chick embryo testes and ovaries. The groups of gonads examined were: A) gonads of normal 18-day-old embryos; B) gonads explants on standard medium; C) standard medium + LH (60 microgram/ml); D) standard medium + FSH (60 microgram/ml); E) standard medium + LH (20 microgram/ml) + FSH (20 microgram/ml). After culture for eight days, the testicular explants from the control group B demonstrated gonocytes in the cortical epithelium, which was stratified and contained no connective tissue, together with a reduction of the diameter of the medullary cords and stromal tissue in relation to 18-day-old embryos. In groups C, D, and E, the cortical epithelium contained no gonocytes and there was differentiation into an albugineous type of connective tissue. The diameter of the cords and the extent of the stroma was similar to that of normal embryos in groups C and D, but were reduced in group E. The cortex in the control group B ovarian explants was thicker than in normal living embryo's ovaries and somatic cells were rarely seen: cords containing gonocytes and lacunae had developed in the medulla. True ovotestes were sometimes observed. The thickness of the cortex and the morphology of the medulla in group C were similar to those of the control group explants. The FSH in the culture medium in groups D and E, caused a reduction in the thickness of the cortex and the development of lacunae.