In summary, the studies reviewed here have indicated which neural functions might be directly influenced by the nucleus basalis. Basalis neurons do not appear to be directly involved in trial-specific memory because, in memory tasks, they have non-differential responses that do not correspond to the information being remembered by the monkey. Similarly, basalis neurons do not appear to be related to movements because, in a go/no-go task, similar neuronal responses occur whether the animal moves or does not move, and, in a delayed response task, different neuronal responses occur during the same arm movement made under different conditions. Basalis neurons also respond differently to the same sensory stimuli presented under different conditions, which indicates that the nucleus basalis is not involved in basic sensory perception. The responses of basalis neurons therefore appear to be strongly influenced by the context or behavioral significance of stimuli. Many basalis neurons respond to appetitive stimuli. In trained animals, the most frequently observed responses have been to a water reward or to stimuli that consistently precede the reward. In naive, thirsty animals, a large proportion of basalis neurons respond to the delivery of water. However, a large number of neurons also respond to an aversive air puff, which indicates that the nucleus basalis cannot be exclusively related to appetitive stimuli. Although some basalis neurons apparently respond only to the appetitive stimulus and others respond only to the aversive stimulus, the majority appear to respond similarly to both stimuli. In particular, almost all of the neurons whose response magnitudes covary with the volume of the water respond similarly to the air puff. Hence, the neurons that appear most likely to be related to the appetitive component of the water are also responsive to an aversive stimulus. Basalis neurons may therefore be related to some common characteristic of aversive and appetitive stimuli, such as the arousing quality of these stimuli. The hypothesis that most basalis neurons are particularly responsive to arousing stimuli could account for the abundance of responses to rewards and stimuli associated with rewards. These phasic responses of basalis neurons are hypothesized to be related to a transient increase in the cortical activation component of arousal, just as the tonic activity of basalis neurons appears to be related to sustained cortical activation.