The influence of high density lipoproteins (HDL) on luteinizing hormone-stimulated rat ovarian theca/interstitial cell steroidogenesis was studied. Without HDL the cells produced primarily androgens from progestin precursors. In the presence of rat or human HDL steroid output increased 3-5-fold, but the type of steroid produced was dependent on the source of the HDL. Human HDL nonselectively amplified luteinizing hormone-stimulated steroid production, whereas rat HDL promoted progestin production without a concomitant increase in androgen output. Comparisons of the activities of apoprotein E-rich HDL (e.g. HDL from intact mature rats) with apoprotein E-poor HDL (e.g. human HDL or rat HDL from hypophysectomized immature rats) suggested that apoprotein E was responsible for the inhibition of androgen production. Furthermore, the inhibitory activity of rat HDL was abolished by depleting apoprotein E-containing lipoproteins with heparin affinity chromatography. Direct evidence that apoprotein E was the inhibitory constituent of rat HDL was obtained by showing that isolated lipid-free rat apoprotein E inhibited androgen production, whereas isolated rat apoproteins A-I and A-IV did not. The possible paracrine function of apoprotein E in ovarian follicular maturation of the ovary is discussed.