Ovaries were obtained from sows immediately after slaughter and were morphologically assigned to different stages of the ovarian cycle. Follicular fluid contained in the predominating follicles was analysed for ten steroid hormones by a multiple, simultaneous radioimmunoassay technique. The steroids measured were pregnenolone, progesterone, 20 alpha-dihydroprogesterone, 17 alpha-hydroxyprogesterone, androstenedione, 5 alpha-dihydrotestosterone, dehydroepiandrosterone, testosterone, oestrone and oestradiol-17 beta. The concentrations of the steroids remained relatively low during the luteal stages until the mid-follicular stage when ovaries contained predominantly small to medium-sized (less than 5.0 mm in diam.) follicles. With the exception of 5 alpha-dihydrotestosterone concentrations, which remained low regardless of the size of the follicles or the stage of the cycle, the concentrations of all the steroids were significantly elevated in the transition from the mid- to late follicular stage, a period when the ovaries contained mainly large (6-10 mm diam.) follicles. Follicles at the ovulatory stage exhibited a profound decline in the concentrations of androgens and oestrogens. In contrast, the magnitude of decline in the levels of 3 progestagens, i.e. pregnenolone, progesterone and 17 alpha-hydroxyprogesterone, was much less than that for androgens and oestrogens, while the concentration of 20 alpha-dihydroprogesterone was actually elevated in the ovulatory follicles. The present results agree with those of earlier studies which measured fewer steroids in follicles obtained by repeated and sequential laparotomies of sows during spontaneous cycles. In contrast, these hormone results differ from those using the PMSG/hCG-stimulated ovary, suggesting that such ovaries may not be a completely valid model for ovarian steroid hormone metabolism in the normally cycling sow.