These experiments examined the effects of corticotropin-releasing hormone (CRH) on single-unit electrophysiological activity of locus coeruleus (LC) neurons. As has been reported previously, infusion of CRH into the ventricular system of the brain (i.c.v.) of halothane-anesthetized adult male rats increased spontaneous discharge rate of LC neurons while producing no increase, and possibly a decrease, in sensory-evoked activity. However, when i.c.v. CRH was given to female rats or immature male rats, which had not been studied previously, LC activity was not altered. To attempt to understand this sex and age difference, potential mechanisms by which i.c.v. CRH elevates LC spontaneous activity in adult male rats were examined; in that i.c.v. CRH activates the pituitary-adrenal axis and autonomic nervous system, these response systems were manipulated. Adrenalectomy (with or without corticosterone replacement by pellet) did not affect the ability of i.c.v. CRH to increase LC spontaneous activity in adult male animals, but blockade of sympathetically-mediated autonomic responses, either by chlorisondamine or the beta adrenergic receptor blocker timolol, blocked this increase, indicating that afferent feedback from peripheral autonomic responses was critical for activating LC neurons following i.c.v. CRH. To determine whether CRH neurotransmission might play a role in this feedback pathway, the CRH antagonist alpha-helical CRH (alpha-hCRH) was microinjected into several brain regions including LC prior to i.c.v. CRH. alpha-hCRH microinjected into LC reduced the increase in LC activity caused by i.c.v. CRH; however, blockade of this increase was total when alpha-hCRH was microinjected into the lateral parabrachial nucleus ipsilateral to the LC recording site, suggesting that increased LC activity following i.c.v. CRH is mediated by CRH acting in the parabrachial region. During these studies, it was also observed that microinjection of alpha-hCRH into LC increased LC spontaneous discharge rate; consequently, CRH was microinjected into LC, and produced a dose-dependent decrease in LC spontaneous activity in both male and female rats, which could be blocked by microinjection of alpha-hCRH - these data indicated that the direct influence of CRH on LC neurons is to decrease their spontaneous activity. To reconcile this with the original report that CRH applied to LC neurons increases their activity, one possibility suggested is that the CRH microinjection procedure used in the present study stimulated inhibitory receptors on LC dendrites whereas the original study stimulated excitatory receptors on LC cell bodies. It is concluded that an inhibitory influence of CRH on LC activity is consistent with recent data indicating that decreased LC activity increases anxiety and stress-related responses, but that direct influences of CRH appear rather minor in determining LC neuronal activity in comparison to other inputs to LC such as are seen after i.c.v. CRH infusion.