The enhancement of radiophosphate uptake into the testes of day-old White Leghorn cockerels has been evaluated as a basis for an assay for gonadotropins. The response varies linearly with the logarithm of the gonadotropin dose in the range from about 0.5 to 50 ug for luteinizing hormone (LH) and follicular stimulating hormone (FSH). LH, FSH, pregnant mare serum (PMS) and human chorionic gonadotropin (HCG) all cause this response. The smallest doses detectable reliably are about 0.1 IU of HCG, 0.05 IU of PMS and about 0.5-1.0 [mu]g of the Armour standard LH and FSH preparations. The assay is not affected by a number of other pituitary and other hormones. Serum does not enhance the response, and no augmentation was found between FSH and LH. The procedure also permits the simultaneous determination of thyroid stimulating hormone (TSH).