The pharmacokinetics and metabolism of moxestrol have been compared in the rat, dog and monkey (rhesus and baboon) and, in some instances, confronted with data simultaneously obtained for ethynl estradiol and previously obtained in humans. The apparent initial volume of distribution (AIVD) of total radioactivity after i.v. administration was of the order of body volume in all species under study; the AIVD of intact moxestrol was even higher. This is in agreement with moxestrol's low binding to specific and non-specific plasma proteins. The half-life of total radioactivity elimination was 14-18 h in the rat and rhesus monkey, but longer (43 and 78 h, i.v. and oral respectively) in the baboon. In the dog, the elimination phase could not be distinguished from the distribution phase and had a half-life of 2 h. The half-life of unchanged moxestrol elimination was shorter and very similar in the rhesus, baboon and human (6.6, 7.5 and 8.2 h respectively) and only 1.4 h in the dog. Regardless of the route of administration or the species under study, the clearance and elimination rate of unchanged moxestrol were higher than of total radioactivity implying that metabolites and/or conjugation products were eliminated more slowly than intact product from plasma. Orally administered moxestrol was rapidly absorbed in all species. Since clearance of total radioactivity and of moxestrol was faster after i.v. than oral administration, but the radioactivity levels excreted in the urine were identical for the two routes, a significant first-pass-effect probably occurred in the liver. Radioactivity distribution in tissues was examined in the rat. Total radioactivity was higher 24 h after administration of labelled moxestrol than of labelled ethynyl estradiol in endocrine tissues; it was equivalent or less in the other tissues. For all tissues, the elimination rate of moxestrol was greater than, or equal to, that of ethynyl estradiol. In dog urine, the only product identified was moxestrol; in rhesus or baboon monkey urine, the principal metabolites were catechol estrogens, which were also present in appreciable amount in rat bile (as methyl ethers) but were minor metabolites in human urine. Hydroxylation in position 16 occurred in rats and humans only, in position 15 alpha in humans and, to a much lesser extent, in rats and monkeys. Thus, the metabolic profile of moxestrol in rats most closely resembles that in humans.