Using single- and double-labelling immunocytochemistry with antibodies specific for the oestrogen receptor and calcitonin gene-related peptide (CGRP), we have demonstrated oestrogen receptor immunoreactivity in the sexually dimorphic CGRP-immunoreactive (IR) population of the medial preoptic area (MPOA). In the short-term gonadectomised female approximately 80% of preoptic CGRP-IR neurones were immunoreactive for the oestrogen receptor. In short-term gonadectomised males, a small population of CGRP-IR cells was visualised in the MPOA only after colchicine treatment. Approximately 30% of CGRP-IR cells in the male were oestrogen receptor-IR, accounting for 2% of the total population of cells containing oestrogen receptors in this area. In the colchicine-treated female, it is estimated that 10-15% of preoptic oestrogen receptor-IR cells contain CGRP. These results indicate that CGRP is synthesised by preoptic neurones with oestrogen receptors. Furthermore, the identification of oestrogen receptors in the sexually dimorphic CGRP population suggests that these neurones may be directly linked with gonadal steroid-dependent, sex-specific functioning of the MPOA.