We have investigated the possibility that there is a correlation between female sexual receptivity and CNS serotonergic activity. In ovariectomized rats primed with a submaximal steroid regime of 2 micrograms oestradiol benzoate (OB) plus 0.2 mg progesterone (P), so that only a proportion were receptive, the 5-hydroxyindoleacetic acid:5-hydroxytryptamine (5-HT) ratio in the hypothalamus was greater in the receptive compared to the non-receptive rats. When rats were primed with 10 micrograms OB alone, there was no difference in 5-HT activity in the receptive and non-receptive animals. Depletion of hypothalamic 5-HT levels by p-chlorophenylalanine methyl ester (PCPA) inhibited behaviour normally induced by OB plus P and this could be reversed by 5-hydroxytryptophan. PCPA had no effect in animals made receptive by OB alone. This indicates that P, but not OB, exerts its stimulatory effect on sexual behaviour via increasing 5-HT activity. Administration of 5-HT into the lateral, but not 3rd ventricle stimulated sexual behaviour and systemic injection of the selective 5-HT1 and 5-HT2 agonists, Ru 24969 and MK 212, inhibited and stimulated behaviour, respectively. It is suggested that 5-HT has a dual effect on female sexual receptivity acting via different systems, the inhibitory tract acting on 5-HT1 and the stimulatory tract on 5-HT2 receptors.