Anti-schizophrenia agents with improved efficacy and side-effect profiles are required. A dopamine D3 receptor agonist, R-(+)-trans-3,4a,10b-tetrahydro-4-propyl-2H,5H-benzopyrano[4,3- b]-1,4-oxazin-9-ol HCl ((+)-PD 128,907), displayed an atypical antipsychotic profile comparable to that of clozapine. (+)-PD 128,907 blocked stereotypy produced by dizocilpine (MK-801) at 12-fold lower doses than those affecting apomorphine-induced stereotypes in mice and did not produce catalepsy. These effects of (+)-PD 128,907 were stereospecific and were blocked by a D3 antagonist. These data suggest a role for D3 receptors in antipsychotic drug action.