Lipid granules in ovaries of immature rats were confined to the interstitial tissue, and comprised 70% cholesteryl esters and 20% triacylglycerols, the balance being phospholipid and free cholesterol. Following treatment with gonadotropin the interstitial granules disappeared as cholesteryl esters were hydrolysed, but reformed in the follicle as it developed, first in the theca, then in the outer granulosa and finally in the inner cells. The cholesteryl ester: triacylglycerol ratio fell during follicular growth, but in the corpus luteum the ratio in the widely distributed granules was 1:1. Esterified fatty acids in both cholesteryl esters and triacylglycerols became longer and more unsaturated as development progressed. The same progression of granules across the follicles was evident in ovaries of normal adult rats. We concluded that lipid granules in interstitial tissue supplied the substrates for synthesis of new cells in adjacent developing follicles, and those in corpora lutea were a prerequisite for steroidogenic competence.