Leptin is a hormone released from adipose tissue and acts in the brain to play an important role in influencing metabolic and cardiovascular function, and it has sympatho-excitatory effects on cardiovascular organs such as the kidney. Insulin is a hormone produced by the pancreas and can also act centrally to increase sympathetic nerve activity to the kidney. The aim of the present study was to determine whether these hormones together can elicit a much greater increase in renal sympathetic nerve activity (RSNA), mean arterial pressure (MAP) and heart rate (HR) than when given alone. Male Sprague-Dawley rats were anaesthetised using isoflurane (2-5%) in O2. The femoral artery was cannulated for mean arterial pressure (MAP), and heart rate (HR) recording and the femoral vein was cannulated for intravenous administration of urethane to maintain anaesthesia (1.4-1.6 g/kg + top ups of 0.05 ml of a 25% solution, as required). Depth of anaesthesia was monitored by the absence of pedal and corneal reflexes. The renal nerve was carefully dissected. RSNA, mean arterial pressure (MAP) and heart rate (HR) were recorded prior to and for 3 hours following intracerebroventricular (ICV) saline (control, n=5), leptin (7 μg, n=5), insulin (500 mU, n=4) and the combination of both insulin and leptin (leptin was administered 15 minutes after insulin, n=4). Following ICV administration of leptin or insulin alone, RSNA was significantly increased by a maximum of 74% and 62% respectively (P<0.0001 compared to saline). ICV injection of leptin and insulin together significantly increased RSNA by a maximum of 124% compared to either hormone alone (P<0.0001). ICV administration of leptin alone did not significantly increase HR compared to saline. By contrast, insulin alone increased HR significantly compared to saline (P<0.0001). This tachycardic effect of insulin was significantly attenuated when both hormones were administered. MAP was not increased following ICV injection of saline (control), leptin alone, insulin alone and leptin and insulin together and it was not significantly different between groups. Resting levels of MAP and HR prior to the ICV injection of drugs were not significantly different between groups. These findings show that (i) insulin and leptin together enhance RSNA and (ii) leptin appears to attenuate the tachycardic effects of insulin. Therefore, these results suggest that in conditions where both hormones are elevated, such as in obesity, the enhanced effects on RSNA may be contributing to the elevation of RSNA seen in such conditions and this may contribute to the cardiovascular complications that accompany obesity.