As a basis for a bioassay for pituitary gonadotrophins, 32P uptake by testes of 1â dayâold chicks was measured at various hours after the injection of LH and of radiophosphate. The greatest response was obtained when 32P was injected 4 hr after the injection of LH and the testes were excised 2 hr after the 32P injection. The response was specific to LH and FSH among anterior pituitary hormones. A parallelism of the logâdose response curves was found among LH, FSH and the chicken anterior pituitary. Mixtures of LH with FSH showed additive responses. Indices of precision (Îť) were less than 0.30. The 6âhr bioassay based on 32P uptake by the chick testes may be useful as a rapid and reliable assay to measure additively the activity of LH and FSH. (Endocrinology91: 385, 1972)