Neurons containing calcitonin gene-related peptide in the medial preoptic nucleus exhibit the largest neurochemically defined sex difference in the rat preoptic area with a 20-fold difference in cell numbers. The gonadal steroid hormones responsible for this sexual dimorphism have been investigated by examining calcitonin gene-related peptide immunoreactivity in the preoptic area of adult rats receiving a variety of perinatal and adult gonadal steroid manipulations. Cells immunoreactive for calcitonin gene-related peptide were examined in two populations within the preoptic area, one in its ventrolateral aspect and the other located in the lateral division of the medial preoptic nucleus. Cell profile counts estimate numbers of calcitonin gene-related peptide-containing cells in the medial preoptic nucleus of the female to be 22.2 +/- 3.0 cells/section compared with 1.0 +/- 0.2 in the male (P < 0.01). No sex differences existed in the preoptic ventrolateral population of calcitonin gene-related peptide cells (males 4.3 +/- 0.2, females 4.4 +/- 0.6 cells/section). Gonadectomy of male rats on postnatal day 2 resulted in the appearance of a calcitonin gene-related peptide-containing cell population in the medial preoptic nucleus which was indistinguishable from intact female rats (19.3 +/- 2.2 cells/section). Gonadectomy of adult male rats resulted in a modest increase in calcitonin gene-related peptide cell numbers within the medial preoptic nucleus (8.8 +/- 0.4 cells/section) and this was fully reversed by replacement of testosterone (0.7 +/- 0.2 cells/section).(ABSTRACT TRUNCATED AT 250 WORDS)