The organization of neuronal cell bodies in the caudal preoptic area (POA) and rostral anterior hypothalamic area (AH) was studied in Nissl-stained brain sections from adult male and female ferrets. Computer-assisted image-analysis procedures were developed to help estimate the areas of cellular density and the sizes of individual perikarya. At the junction of the POA and AH, a bilateral dorsal-medial group of neurons was apparent only in male ferrets (dorsal nucleus). At the same coronal level, a ventral-medial group of neurons was apparent bilaterally in both males and females (ventral nucleus). The mean somal area of cells in the dorsal nucleus of males was significantly greater than the mean somal area of cells in the corresponding dorsal region of females or in the ventral nucleus of both sexes. The dorsal nucleus was clearly discernible in adult males regardless of their hormonal status, although cells in the dorsal nucleus were larger in intact breeding males or gonadectomized males given testosterone, estradiol or dihydrotestosterone than in gonadectomized males given no gonadal hormones or given progesterone. Neither the grouping of large cells nor the steroid-induced increase in cell size, characteristic of the male dorsal nucleus, was seen in the comparable dorsal region of females. The sex difference in cellular organization observed in the ferret at the junction of the POA and AH is the first difference of this type to be seen in the POA/AH of a non-rodent mammalian species. Its identification will, hopefully, aid in the analysis of the neural mechanisms that control various sex-specific behaviors in this species.