The dopamine (DA) neurons in the ventral tegmental area and medial substantia nigra (VTA/mSN) projecting to the limbic forebrain and prefrontal cortex have long been postulated to play a major role in cognitive and behavioral effects of stress. In this study, the effects of a chronic stressor (prolonged exposure to cold) on the spontaneous activity of DA neurons in the VTA/mSN were examined. Extracellular single-unit recordings of DA neurons were performed in rats following a 17-day continuous exposure to a cold (4 degrees C) environment. Compared to controls, cold-exposed rats displayed 64% fewer spontaneously active DA neurons. The average spike activity (average firing rate, average spikes fired in bursts) of the DA cells that remained active in the cold-exposed rats did not differ significantly from controls. However, a significantly larger proportion of those cells showed excessive burst activity, compared to the DA cell population in controls. These results show that chronic stress can lead to the cessation of spontaneous activity in a subpopulation of VTA/mSN DA cells. These changes may indicate that unlike acute stress, which can potently activate the mesolimbic/mesocortical DA systems, chronic stress leads to an adaptive reduction in the number of active DA cells, perhaps altering the response of these systems to subsequent stressors.