Oestrogen is known to have protective effects at the level of the heart and blood vessels via its actions on the α and β subtypes of the oestrogen receptor (ER). It has also been reported to have pronounced effects on the activity of the autonomic nervous system thereby influencing cardiovascular function (Saleh & Connell, 1999, 2000). To date, only one isoform of the ERα subtype has been cloned, whilst 5 splice-variant isoforms of the ERβ subtype have been identified: β1, β2, β1δ3, β2δ3 and β1δ4 (Maruyama et al. 1998; Peterson et al. 1998). This study aims to investigate the presence of ER subtypes in the brainstem and the nucleus of the solitary tract (NTS), one of several medullary nuclei known to be involved in cardiovascular regulation, of both male and female Wistar rats. Adult Wistar rats (150-200g) were killed and tissue samples removed. RNA was extracted and reverse transcribed to cDNA. The presence of the subunits was examined using PCR with subtype specific primers. Levels of subtype expression were determined using real-time PCR with Taqman gene expression assays (ABI 7500). The veracity of the PCR products was verified by DNA sequencing. The data suggest that the ERα, β1 and β2 subtypes are strongly expressed in the brainstem of both male and female rats (n=6). In addition, the ERβ splice variants β1δ3, β2δ3 and β1δ4 were also detected, although the expression of β1δ4 was much weaker than the other subtypes. Results from the male and female NTS samples suggest the presence of both ERα and ERβ. PCR analysis of ER subtype expression in the brainstem was also investigated at various stages of the oestrus cycle of the female rat. Oestrus stages were determined histologically and the cycle split into metoestrus, dioestrus, proestrus and oestrus. Expression of all the subtypes was present in each stage; however, real-time PCR data using ERα and ERβ taqman probes suggest that the levels of these two subtypes change throughout the cycle peaking in the metoestrus and dioestrus stages. The results of this study suggest that gonadal steroid hormones may modulate cardiovascular function via a variety of ER subtypes expressed on brainstem autonomic neurons, in both male and female rats.