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Spike activity was studied in 95 neurons in the basal magnocellular nucleus in rabbits during spontaneous behavior and during performance of a conditioned operant response. Nearly half the neurons (48.4%) showed significant (p <0.05) negative correlations between spontaneous discharges and the power of the frontal lobe EEG delta rhythm; most of these cells could be identified as cholinergic projection neurons. Neurons of this group had predominantly excitatory responses to the conditioned stimulus during performance of the operant task, while the responses to the conditioned stimulus of presumptively non-cholinergic neurons, not projecting to the cortex, were mainly inhibitory. The activatory responses of neurons in the basal magnocellular nucleus to the conditioned stimulus were markedly stronger while the animals performed the operant response as compared with performances in which there was no response to the conditioned stimulus. These results provide evidence that the basal magnocellular nucleus supports the level of waking and attending required for performance of operant conditioned reflex activity.