The response of noradrenergic (NE), serotonergic (5-HT), and dopaminergic (DA), neurons to repeated presentations (once/2 s for 64 trials) of phasic auditory (click) and visual (flash) stimuli was examined in freely moving cats. All 3 groups of neurons displayed similar response latencies and somewhat similar durations of excitation to both stimulus modalities. 5-HT neurons in the mesencephalic raphe nuclei showed no decrease in responsiveness across trials to either of the stimuli. DA neurons in the substantia nigra displayed no decrease in responsiveness across trials to the auditory stimulus, but did display an approximately 50% decrease in response to the visual stimulus. NE neurons in the locus coeruleus showed an approximately 50% decrease in responsiveness to both the auditory and visual stimuli. These data are consistent with previous studies showing that the response of many neurons in the brainstem reticular formation habituates to the repetitive presentation of sensory stimuli. They also show that the response of reticular formation neurons are heterogeneous and that they can be subdivided on the basis of their neurochemical identity. Finally, these data provide support for the involvement of NE neurons, and to a lesser extent DA neurons, in various forms of behavioral plasticity.