Phosphodiesterase 10A (PDE10A) is a recently identified cyclic nucleotide phosphodiesterase expressed primarily in dopaminoreceptive medium spiny neurons of the striatum. We report that papaverine is a potent, specific inhibitor of PDE10A and use this compound to explore the role of PDE10A in regulating striatal function. Papaverine administration produces an increase in striatal tissue levels of cGMP and an increase in extracellular cAMP measured by microdialysis. These cyclic nucleotide changes are accompanied by increases in the phosphorylation of CREB and ERK, downstream markers of neuronal activation. In rats, papaverine potentiates haloperidol-induced catalepsy, consistent with the hypothesis that inhibition of PDE10A can increase striatal output and prompting a further evaluation of papaverine in models predictive of antipsychotic activity. Papaverine is found to inhibit conditioned avoidance responding in rats and mice and to inhibit PCP- and amphetamine-stimulated locomotor activity in rats. The effects of papaverine on striatal cGMP and CREB and ERK phosphorylation, as well as on conditioned avoidance responding, were absent in PDE10A knockout mice, indicating that the effects of the compound are the result of PDE10A inhibition. These results indicate that PDE10A regulates the activation of striatal medium spiny neurons through effects on cAMP- and cGMP-dependent signaling cascades. Furthermore, the present results demonstrate that papaverine has efficacy in behavioral models predictive of antipsychotic activity. Thus, inhibition of PDE10A may represent a novel approach to the treatment of psychosis.