The effects of 5-HT(2) receptor ligands on the performance of rats were investigated using a 5-choice serial reaction time (5-CSRT) task. Systemic administration of DOI (0.03 to 0.3 mg/kg subcutaneously [SC]), a 5-HT(2) receptor agonist, did not impair choice accuracy of well-performing rats under either baseline conditions or more demanding conditions of the task, in which the stimulus duration or intensity were reduced or the intertrial interval (ITI) was decreased. DOI (0.1 mg/kg or 0.15 mg/kg) increased premature responding (the probability of intertrial interval hole pokes) in all testing conditions, except under conditions of a short ITI when the rats did not make any hole responses. Ketanserin (0.1 to 0.3 mg/kg SC), a 5-HT(2A) receptor antagonist, had no marked effect on performance. When combined with ketanserin (0.2 mg/kg SC), however, DOI (0.1 mg/kg) did not increase premature responding. The lowest doses of DOI (0.05 and 0.1 mg/kg) that increase premature responding had no effect on open-field performance. Further, the effects of systemically administered DOI were not reproduced by bilateral administration of DOI into the anterior cingulate cortex. These data indicate that excessive activation of 5-HT(2A/2C) receptors interferes with response control rather than visual attention. Furthermore, the DOI-induced enhancement of impulsive responses are not due to locomotor hyperactivity, and the anterior cingulate cortex is not the primary site of action for this enhancement of premature responding.