During the 26-h laying cycle of the hen the three sex steroids, progesterone (P4), estrogen (E), and testosterone (T), show peaks and valleys in both the follicle wall and the peripheral plasma. Furthermore, for E and T (but not for P4) at comparable times, there is an inverse relationship between the size of the follicle and the concentration of these hormones. Shortly before ovulation all three steroids peak in synchrony, at other times only E and T rise and fall together while P4 shows no change. The question arose whether similar changes in steroid synthesis can be produced by exogenous LH. A single injection of 25 mug of ovine LH was given to laying hens at a time of the laying cycle when no natural peaks of the steroids are seen. The hens were killed at 5, 30, or 90 min after injection and the walls of the three largest follicles as well as the plasma of these hens were assayed by RIA for P4, T, and E. Both the follicle walls and the plasma showed highly significant (P smaller than 0.005) increases in T and P4 but not in E, even though E does show significant changes during the normal cycle. The reason for this difference in response of the steroids is being investigated. The 3 largest follicles respond to exogenous LH in the same size order as do untreated hens during the cycle. The conclusion is drawn that peaks of T and P4 normally seen are due to endogenous releases of LH while E seems to be controlled differently.