Previous reports have shown that dopamine (DA) in the zona incerta (ZI) has a stimulatory effect on gonadotrophin release. We have now investigated the possibility that steroids exert their feedback effects on the release of luteinizing hormone (LH) via catecholamine systems in the ZI. Since the same steroid regimes also stimulate female sexual behaviour, the possibility that the ZI is also involved in the control of sexual activity was investigated. Lesions in the ZI increased proceptive and receptive behaviour in oestrogen-primed ovariectomised rats that exhibited a low level of lordosis in a pre-lesion test and had no effect in receptive animals. Turnover rates (TR) of DA, noradrenaline (NA) and adrenaline were measured in the ZI, preoptic area (POA), arcuate nucleus (ARC), median eminence (ME) and ventromedial nucleus in ovariectomised rats treated 54 h before with either oil, oestradiol benzoate (OB) at 5, 10 or 50 µg/rat, or 5 µg/rat OB followed by 0.5 mg/rat progesterone (P) 48 h later. The TR as measured from the decline in concentration after α-methyltyrosine and changes in DOPAC concentration were correlated with the effect of the steroids on plasma LH and lordotic activity. Confirming previous reports, NA turnover in the POA and ME was increased, and DA turnover in the ME was decreased by steroids when they enhanced LH release. DA turnover in the ARC and ME was reduced when lordosis behaviour increased. Treatment with OB plus P stimulated LH release and sexual receptivity and at the same time significantly increased DA and NA turnover in the ZI. There was no correlation between these parameters after OB alone. This report shows that the DA system in the ZI may mediate the stimulatory effect of P on LH release in OB-primed rats but is not involved in the feedback effects of oestradiol alone. NA activity in the ZI is increased after OB plus P and may therefore also be concerned with stimulating LH release. The ZI is involved in the control of sexual behaviour, as electrolytic lesions in this area enhance receptivity and proceptivity, but the catecholamine systems in the ZI do not appear to mediate this control.