Because of evidence that the nucleus accumbens mediates the activating effects of many drugs, this study examined the hypothesis that the firing rates of individual nucleus accumbens neurons are positively correlated with spontaneous changes in behavioral arousal that occur during the sleep-wake cycle. The present report examined the firing patterns of 80 neurons in the nucleus accumbens of unanesthetized, unrestrained rats during various electrographically determined levels of arousal. Synaptic responses to stimulation of hippocampal and pallidal nucleus accumbens afferents indicated that the present sample of neurons was similar to a large population of nucleus accumbens neurons previously recorded in anesthetized rats. Confirming the participation of the nucleus accumbens in behavioral arousal, the firing rates of nucleus accumbens neurons were greatest during wakefulness and rapid eye movement sleep and lowest during non-rapid eye movement sleep. Furthermore, the induction of halothane anesthesia decreased behavioral and electrocorticographic arousal concurrent with a suppression of the spontaneous nucleus accumbens unit discharge. These data support the hypothesis that the firing of nucleus accumbens neurons is closely related to arousal.