Although manipulation of second messenger systems is widespread in cell biology, there are few experiments examining the consequences of such manipulation on behavior. In three separate experiments, we extended earlier work by Miller and Kelly (1975) that examined the behavioral effects of microinfusion of cholera toxin (CTX) into the nucleus accumbens (N. Acc.) in rats. CTX is a bacterial toxin that ADP-ribosylates the Gs transducer protein and stimulates production of cAMP. For Experiment I, three groups of rats received either saline or CTX (50 or 500 ng/microliter) into the N. Acc. Locomotor activity was measured for 4 hr following a single CTX infusion and subsequently for 4 hr on 6 consecutive days. No acute effects on motor activity were observed. However, the 500 ng dose of CTX induced long-lasting hyperactivity that was apparent 24 hr later and that lasted 4 d. A smaller but significant hypermotility occurred on days 4 and 5 following infusion of the 50 ng dose. Site specificity of this effect was investigated in Experiment II by infusion of CTX (250 ng/microliter) into either the N. Acc. or the posterior dorsal striatum (PDS). CTX treatment of the PDS had no behavioral effects while the long-lasting hyperactivity following treatment of the N. Acc. was replicated. In Experiment III the effect of intra-accumbens pretreatment with saline or CTX (10 ng/microliter) on d-amphetamine (0.5 mg/kg, i.p.)- and cocaine (7 mg/kg, i.p.)-induced motor activity was investigated.(ABSTRACT TRUNCATED AT 250 WORDS)