Ovaries from 15-day-old mice were cultured with or without fsh and lh for 3 or 6 days. Paired control ovaries were fixed at the time of dissection. The number of follicles in each ovary having at least three layers of granulosa cells was determined, and changes in the follicle population during culture were examined. The diameter of each follicle with at least four layers was measured. fsh stimulated follicle growth throughout the experiment but lh stimulated it only between Day 3 and Day 6. lh significantly increased the proportion of follicles with diameters exceeding 119 μ on Day 6; fsh tended to increase this proportion but its effect was not significant. lh reduced the incidence of pycnotic nuclei in peripheral follicles but fsh had no such effect. It is suggested that lh stimulates follicle growth by stimulating production of an oestrogen which must accumulate to a critical level before becoming effectively mitogenic.