The activity of dopamine-containing ventral tegmental area (VTA) units was recorded by means of movable 32- and 64-microns-diameter insulated Nichrome wires in freely moving cats. The VTA units displayed a slow, somewhat irregular activity during quiet waking (mean 3.63 +/- 0.41 spikes/s) and showed no significant change in activity during slow-wave sleep or REM sleep. Although VTA unit activity was somewhat higher and more erratic during active waking, there was no relationship between unit discharge and phasic movement. These neurons were inhibited (-87%) by small doses of apomorphine (1.0 mg/kg, i.p.) and excited (+43%) by small doses of haloperidol (0.5 mg/kg, i.p.). The stress of a conditioned emotional reaction (CER) paradigm resulted in a significant increase in the discharge rate of VTA neurons (+39%), compared with the quiet-waking baseline. The CER paradigm increased plasma glucocorticoids by 74%. Neurochemical studies revealed that the CER paradigm resulted in a significant decrease of dopamine in the limbic forebrain (-31%), whereas both homovanillic acid (+47%) and dihydroxyphenylacetic acid (+43%) concentrations were increased. No significant changes in dopamine metabolism were observed in the striatum under the CER situation. These data have implications in relation to the role of stress and dopamine in mediating certain psychiatric disorders.