The dopamine receptor antagonists SCH 23390 and spiperone show highly selective in vitro affinity for D-1 and D-2 dopamine receptor subtypes, respectively. We studied the effects of these selective antagonists on the supersensitive locomotor response to apomorphine in rats following 6- hydroxydopamine (6OHDA) lesions of the nucleus accumbens (N. Acc.). Both D-1 and D-2 receptor antagonists produced dose-dependent blockade of the supersensitive locomotor response at doses that did not depress baseline locomotor activity. The behavioral properties of these D-1 and D-2 receptor antagonists were further examined using a simple step-down motor task. Both antagonists produced catalepsy as evidenced by dose-dependent increases in step- down latency. These results indicate that drugs with distinct in vitro dopamine binding affinities cannot be distinguished on the basis of their ability to inhibit supersensitive locomotor activity or simple motor tasks in rats in vivo.