Analysis of recordings of single neuronal activity in the striatum of monkeys engaged in behavioural tasks has shown that tonically active neurons (TANs) can be distinguished by their distinct spontaneous firing and functional properties. As TANs are assumed to be cholinergic interneurons, the study of their physiological characteristics allows us to gain an insight into the role of a particular type of local-circuit neuron in the processing of information at the striatal level. In monkeys performing various behavioural tasks, the change in the activity of TANs, unlike the diversity of task-related activations exhibited by the phasically active population of striatal neurons, involves a transient depression of the tonic firing related to environmental events of motivational significance. Such events include primary rewards and stimuli that have acquired a reward value during associative learning. These neurons also respond to an aversive air puff, indicating that their responsiveness is not restricted to appetitive conditions. Another striking feature of the TANs is that their responses can be modulated by predictions about stimulus timing. Temporal variations in event occurrence have been found to favour the responses of TANs, whereas the responses are diminished or abolished in the presence of external cues that predict the time at which events will occur. These data suggest that the TANs respond as do detectors of motivationally relevant events, but they also demonstrate that these neurons are influenced by predictive information based on past experience with a given temporal context. TANs represent a unique subset of striatal neurons that might serve a modulatory function, monitoring for temporal relationships between environmental events.