The pertussis toxin (PTX)-sensitive G proteins Gi and G(o) may be implicated in drug reinforcement and addiction, since certain reward- related dopamine and opiate receptor subtypes are coupled to these G proteins, and since chronic exposure to cocaine or morphine alters levels of these G proteins in the nucleus accumbens (NAc). As a direct test of this hypothesis, Gi and G(o) proteins in the NAc were selectively inactivated by intra-accumbens injections of PTX in rats self-administering either cocaine or heroin. In control animals, bilateral injections of inactive PTX (0.1 microgram/1 microliter/side) in the NAc failed to alter baseline rates of cocaine and heroin self- administration. In contrast, the same dose of active PTX produced significant, long-lasting increases (up to 1 month) in the self- administration of both drugs, and shifted the dose-response curves to the right. These results suggest that PTX reduces or shortens the reinforcing efficacy of cocaine and heroin, leading to compensatory increases in drug self-administration. Similar NAc injections of PTX reduced the level of Gi alpha and G(o) alpha subunits as measured by both ADP-ribosylation and Western blot, without affecting levels of Gs alpha or G beta subunits. The effect of the toxin was mainly limited to the NAc, and no evidence of abnormal cell death or gliosis was observed. The onset of changes in self-administration rate coincided with the onset of changes in ADP-ribosylation, suggesting that, initially, the increased drug self-administration results directly from a reduction in functional Gi and G(o) proteins. After 28 d, self- administration baselines began to recover while levels of G protein ADP- ribosylation and immunoreactivity remained low, suggesting that adaptive mechanisms are involved at later time points. These results provide direct support for a common role of Gi and G(o) proteins in the NAc in the reinforcing and addictive properties of psychostimulant and opiate drugs.