A possible role for G proteins in contributing to the chronic actions of cocaine was investigated in three rat brain regions known to exhibit electrophysiological responses to chronic cocaine: the ventral tegmental area, nucleus accumbens, and locus coeruleus. It was found that chronic, but not acute, treatment of rats with cocaine produced a small (approximately 15%), but statistically significant, decrease in levels of pertussis toxin-mediated ADP-ribosylation of Gi alpha and Go alpha in each of these three brain regions. The decreased ADP-ribosylation levels of the G protein subunits were shown to be associated with 20-30% decreases in levels of their immunoreactivity. In contrast, chronic cocaine had no effect on levels of G protein ADP-ribosylation or immunoreactivity in other brain regions studied for comparison. Chronic cocaine also had no effect on levels of Gs alpha or G beta immunoreactivity in the ventral tegmental area and nucleus accumbens. Specific decreases in Gi alpha and Go alpha levels observed in response to chronic cocaine in the ventral tegmental area, nucleus accumbens, and locus coeruleus are consistent with the known electrophysiological actions of chronic cocaine on these neurons, raising the possibility that regulation of G proteins represents part of the biochemical changes that underlie chronic cocaine action in these brain regions.