The sympathetic nervous system is widely assumed to respond to physiological challenges in a global and generalized manner. However, a number of observations have highlighted regional variations in the pattern of sympathetic outflow consequent to external stimuli, including electrical stimulation, thermal stimulation, acute exposure to hypoxia and injection of drugs (Futuro-Neto HA & Coote JH. 1982; Morrison SF. 2001). Since the majority of previous studies on the regional differences in sympathetic outflow have been generated in acutely prepared anesthetized animals, little is known as to whether the patterning of sympathetic outflow occurs in a regionally different manner during daily activity in the conscious animal. We have recently developed a method to measure renal and lumbar sympathetic nerve activity simultaneously and continuously in freely moving rats. This has been utilized to examine the regional differences in sympathetic outflow which occur as a result of natural behavioural activity in rats. Further, the functional relationships between the behavioural related changes in sympathetic outflow and cardiovascular function were evaluated. Wistar rats were used for all experiments. All procedures were undertaken in accordance with National and International guidelines. Rats were operated on in two stages. During the first surgical procedure, the electroencephalogram (EEG), electrocardiogram (ECG), and electromyogram (EMG) electrodes were implanted. At least five days after the first surgery, the following were implanted: electrodes for the measurement of renal sympathetic nerve activity (RSNA) and lumbar sympathetic nerve activity (LSNA); catheters for the measurement of systemic arterial pressure (Pa) and venous pressure (Pcv); Doppler flow cuffs for the measurement of iliac, mesenteric and renal blood flows. Three days after the second surgery, recordings were begun in a sound attenuated, temperature (24°C) controlled chamber. Behavioural states were scored by standard criteria based on EEG and EMG as well as behavioral observations noted at the time of data collection. The animal’s behaviour was classified as rapid eye movement (REM) sleep, non-REM (NREM) sleep, quiet awake, moving and grooming states. Interestingly, a key observation was that there was a regional diversity in sympathetic outflow during REM sleep. Thus, RSNA decreased in a step manner during REM sleep that was accompanied by a step increase in LSNA. This indicated that the sympathetic regulation did not consist of a simple unidirectional readjustment, indeed, the renal sympatho-inhibition coexisted with a sympatho-excitation in the nerves to the muscle during REM sleep. This was reinforced at the functional level in that mesenteric and renal blood flows and vascular conductances were increased while iliac blood flow and vascular conductance was decreased in REM sleep. These changes were accompanied by an increase in systemic arterial pressure at a time when heart rate was decreased, which could be partly explained by the regional diversity in sympathetic outflow; thus, the onset of REM sleep may cause an increased resistance in the muscle vascular bed which is able to partly compensate the decreased vascular resistance in the visceral organs and the reduction in cardiac output associated with the decrease in heart rate. A further point of interest was the finding that during the other behavioural states including NREM, quiet awake, moving, and grooming state, both LSNA and RSNA increased in a parallel manner and in proportion to the rise in physical activity level. At the same time, systemic arterial pressure increased and was associated with an elevation in heart rate. The increases in LSNA and RSNA, which occurred together with the rise in heart rate, would cause the vasoconstriction of the visceral organs and the non-active muscle, which would contribute to the elevation in systemic arterial pressure. Together, these findings demonstrate that during REM sleep and other behavioural states, different patterns of activity occurred in the RSNA and LSNA. This differential response in sympathetic outflow can be related to state-related changes in organ and cardiovascular function during natural behaviour in rats.