SCH 23390 and SK&F 83566 have been utilized as selective antagonists at postjunctional dopamine receptors. However, in the isolated rabbit thoracic aorta evidence for competitive antagonism of serotonin was obtained. The KB values were 11 and 34 nM for SK&F R-83566 and SCH 23390, respectively. The S-enantiomer of SK&F 83566 was a weaker antagonist at the vascular serotonin receptor (KB = 1.5 microM). Thus, these results indicate that SCH 23390 and SK&F 83566 are not totally specific for the dopamine DA-1 receptor because they can also be potent antagonists at the vascular serotonin receptor.