Single unit activity of dopaminergic neurons in the substantia nigra was recorded in freely moving cats under a variety of conditions. These neurons displayed their highest discharge rate during active waking (3.68 +/- 0.30 spikes/s), which was 20% greater than their discharge rate during quiet waking (3.07 +/- 0.20). Although these cells fired somewhat faster during active waking, their activity displayed no correlation with phasic EMG changes, and, in general, their activity showed little relationship to overt behavioral changes. As the cat progressed from quiet waking through slow-wave sleep and REM sleep there was no significant change in either the rate or pattern of firing of dopaminergic neurons. In addition, no correlation was observed between the activity of these neurons and either sleep spindles or PGO waves. These neurons did respond, however, to the repeated presentation of a click or light flash with excitation followed by inhibition, with no evidence of habituation. One of the most impressive changes in dopaminergic unit activity was a large decrease in association with orienting responses. This was seen in over 50% of the cells in which this relationship was examined. As the behavioral orientation habituated with repeated stimulus presentation, so did the associated dopaminergic unit suppression. In conclusion, dopaminergic neurons maintain a remarkably constant rate and pattern of firing across a variety of behaviors and states. However, this stability can be dramatically altered under special circumstances, such as during and following orienting responses.