The effects of clozapine, thiothixene, sulpiride, chlorpromazine and loxapine were examined in pigeons responding under a delayed matching-to-sample (DMTS) procedure using 0-, 2- and 8-sec delay intervals. Chlorpromazine (3-100 mg/kg), thiothixene (0.03-1.7 mg/kg), clozapine (0.1-5.6 mg/kg) and loxapine (0.1-10 mg/kg) produced dose-related decreases in the percent of correct responses (accuracy). With the exception of chlorpromazine, the relative magnitude of the accuracy-decreasing effects were unrelated to the length of the delay interval and the nondrug levels of accuracy. In contrast to these accuracy-decreasing effects, sulpiride (3-300 mg/kg) failed to decrease accuracy across the range of doses evaluated. Chlorpromazine, loxapine and clozapine increased response rates at low doses and then decreased response rates as the dose was increased. Thiothixene and sulpiride only decreased response rates in a dose-dependent fashion. The order of potency for the rate-suppressing effects of these drugs was thiothixene greater than clozapine = loxapine greater than chlorpromazine greater than sulpiride. The results of the present investigation suggest that, despite similar dopamine antagonist properties, neuroleptics produce qualitatively different effects in pigeons responding under DMTS procedures.