The effect of sensory stimulation on discharge in norepinephrine-containing locus coeruleus (NE-LC) neurons was studied in unanesthetized behaving rats. Single unit and multiple unit extracellular recordings demonstrated consistent patterns of response. (1) Short latency (15-to 50-msec), transient, biphasic changes in discharge were elicited predictably by non-noxious auditory, visual, and somatosensory stimuli; individual recordings typically exhibited polysensory responsiveness. (2) Sensory-evoked field potentials (FPs) were synchronized with unit responses simultaneously recorded from the same electrodes. (3) The magnitudes of sensory-evoked response varied as a function of vigilance, so that the largest responses occurred for stimuli which awakened animals and the least responsiveness was exhibited during uninterrupted sleep. (4) Sensory responsiveness decreased during grooming and sweet water consumption, similar to the results for sleep. (5) Characteristic response properties were topographically homogeneous throughout the NE-LC. (6) Discharge was synchronized markedly among neurons in multiple unit populations during phasic robust responses. These results are interpreted in light of the preceding report (Aston-Jones, G., and F.E. Bloom (1981) J. Neurosci. 1: 876-886) and studies of the postsynaptic effects of NE to indicate that the NE-LC system may function more in phasic processes than in modulation of the tonic arousal level. We propose that pronouced NE-LC discharge may enhance activity within target cell systems primarily concerned with processing salient external stimuli and suppress cental nervous system activity related more to tonic, vegetative functions. Thus, the NE-LC system may bias global behavioral orientation between stimuli in the external versus internal environments.