Chicks, one—day old, were injected with LH, FSH, and LH + FSH for two days and the levels of microsomal protein, cytochrome P— 450, and 17α-hydroxylase were measured in the testis. Since LH and FSH act on different cells in the testis, the levels of cytochrome P-450 and 17α—hydroxylase are reported both per mg microsomal protein and per chick. Between a dose range of 3–100 μg of LH, the levels of cytochrome P-450, 17α—hydroxylase, and microsomal protein were found to increase as a linear function of the log dose of LH. LH at the 100 μg dose level was found to increase, per mg microsomal protein, the levels of cytochrome P-450 and 17α—hydroxylase by 200% and 80%, respectively. On a per chick basis, these increases were 450% and 250%, respectively. FSH at the 100 μg dose level was found to decrease, per mg microsomal protein, the levels of cytochrome P-450 and 17α—hydroxylase by 27% and 48%, respectively. On the per chick basis, increases in the level of 61% and 30% were found after correction for a 1% contamination by LH. The LH stimulation of the levels of cytochrome P-450 and the 17α—hydroxylase was 7–8 times that of FSH. The dose response of the cytochrome P-450 dependent hydroxylase to LH suggests that the level of this enzyme plays an important role in the physiological control of androgen biosynthesis. {Endocrinology 91: 1506, 1972)