Rams and ewes show a negative-feedback response to peripheral treatment with testosterone, with both sexes having a similar degree of suppression in luteinizing hormone (LH) secretion during the breeding season. At least part of the action of testosterone to suppress gonadotropin-releasing hormone/LHsecretion is exerted via interaction with an androgen receptor. The distribution of androgen receptor containing cells in the hypothalamus has been described for the ram, but similar studies have not been performed in the ewe. In the present study, we tested the hypothesis that levels of androgen receptor mRNA expression in the preoptic area and hypothalamus would be similar in rams and ewes. Perfusion-fixed brain tissue was obtained from adult Romney Marsh ewes (luteal phase) and rams during the breeding season (n Â¼ 4/sex). Androgen receptor mRNA expression was quantified in hypothalamic sections by in situhybridization using an 35S-labelled riboprobe and image analysis. Hybridizing cells were found in the medialpreoptic area, bed nucleus of the stria terminalis, anterior hypothalamic area, ventromedial nucleus, arcuatenucleus and premamillary nucleus. The level of androgen receptor mRNA expression was higher in rams than ewes in the rostral preoptic area, caudal preoptic area and rostral portion of the bed nucleus of thestria terminalis, with no sex difference in other regions. The preoptic area and bed nucleus of the striaterminalis are important for reproductive behaviour and the sex differences in androgen receptor mRNA expression at these levels may relate to this. The high level of androgen receptor mRNA expression in the basal hypothalamus, with no sex difference, is consistent with the role of this region in the regulation of gonadotropin secretion.