Previous immunocytochemical analyses have identified a substantial, gonadal steroid-dependent sex difference in the number of cells expressing calcitonin gene-related peptide (CGRP) in the rat preoptic area. Using three 35S-labelled antisense oligonucleotide probes specific for both alpha and beta CGRP, the present study has examined CGRP mRNA expression within the medial preoptic nucleus (MPN) of intact and gonadectomised male and female rats. Cells expressing CGRP mRNA were found to be more numerous in the intact female (21 +/- 2 cells/hemisection) compared with the male (6 +/- 1; P < 0.01) although the average CGRP mRNA content of MPN cells was not different between intact males (62 +/- 7 silver grains/cell) and females (69 +/- 6 silver grains/cell). Gonadectomy resulted in a significant increase in the number of CGRP mRNA expressing cells detected in the male (12 +/- 1 cells/hemisection; P < 0.01) and an increase (P < 0.05) in the mean CGRP mRNA content per cell in both males (99 +/- 12 silver grains/cell) and females (107 +/- 11 silver grains/cell). These results show that sex differences exist in the number of cells containing CGRP transcripts in the rat MPN although average CGRP mRNA content per cell is not different between males and females. Gonadal steroids appear to exert a tonic suppressive influence on the CGRP mRNA content of MPN cells in both males and females.