The functional relationship between sexually dimorphic neural populations and sex differences in reproductive functioning is unclear. The present study has investigated the function of the sexually dimorphic, estrogen-receptive, calcitonin gene-related peptide (CGRP) neurones in the female preoptic area by examining patterns of Fos immunoreactivity within these cells in relation to the luteinizing hormone surge and lordosis behaviour. In the first experiment, ovariectomized rats were treated with estradiol alone or estradiol plus progesterone to induce the luteinizing hormone surge. The percentage of CGRP neurones with Fos-positive nuclei was not different in estradiol alone (18 +/- 4%) and estradiol/progesterone-treated (24 +/- 3%) rats although the number of Fos-immunoreactive cells in the medial preoptic nucleus was increased 2-fold (P < 0.01) in estrogen/progesterone-treated rats and 40 +/- 5% of luteinizing hormone-releasing hormone neurones were found to express Fos in this group. In the second experiment, ovariectomized rats were treated with estradiol and progesterone and either, mated with a single male or placed in an empty cage, for 30 min. The number of Fos-immunoreactive cells in the medial preoptic nucleus was increased 4-fold in mated rats (P < 0.01) and the percentage of CGRP neurones with Fos-positive nuclei increased from 24 +/- 3% to 38 +/- 2% (P < 0.01) in mated animals. No differences were detected in the number of luteinizing hormone-releasing hormone neurones with Fos-positive nuclei in mated and non-mated animals.(ABSTRACT TRUNCATED AT 250 WORDS)