Leptin is an adipose tissue-derived cytokine hormone, which reduces body weight via interactions with hypothalamic neurones. Leptin receptors capable of activating the JAK-STAT signal transduction pathway are expressed at high levels in the hypothalamus, particularly in the arcuate nucleus. In order to identify the chemical mediators of leptin's action in the hypothalamus, we have examined whether GABA neurones of the hypothalamic arcuate nucleus contain leptin receptors and the leptin-activated transcription factor STAT3. GABAergic neurones, as visualized by antisera to the GABA-synthesizing enzyme glutamic acid decarboxylase (GAD) and GABA, were demonstrated in the ventromedial and ventrolateral parts of the arcuate nucleus. GABA neurones in the ventromedial arcuate nucleus were shown to contain leptin receptor immunoreactivity, as revealed using an antiserum generated to a sequence common to all isoforms of the leptin receptor (Ob-R), as well as an antiserum generated to the carboxy-terminal end of the long leptin receptor (Ob-Rb), and immunoreactivity for the leptin-induced signal transduction molecule STAT3. Ventromedial GABA neurones were also shown to contain neuropeptide Y, whereas ventrolateral proopiomelanocortin-containing neurones lacked GAD and GABA immunoreactivity. Levels of mRNA for GAD65, GAD67 and the vesicular GABA transporter (VGAT) were analysed in the arcuate nucleus of leptin-deficient ob/ob mice and lean control mice by in situ hybridization. No significant differences in GAD65, GAD67 or VGAT mRNA were detected in the arcuate nucleus of ob/ob mice as compared to lean control mice. The presence of leptin receptor and STAT3 in GABAergic arcuate neurones, but absence of changes in gene transcription for GAD and VGAT mRNA suggests, that leptin does not transcriptionally regulate the expression of proteins involved in GABAergic transmission in arcuate neurones. However, mechanisms other than transcriptional regulation for leptin to influence arcuate GABA neurones may exist.