The discriminative stimulus properties of the N-methyl-D-aspartate (NMDA) antagonist MK-801 ((+)-5-methyl-10,11-dihydro-5H-dibenzo(a,d)cyclohepten-5,10-imine) were investigated in rats trained to discriminate phencyclidine (PCP; 1.25 mg/kg i.p.) from saline on a standard two-lever fixed ratio 32 schedule of food reinforcement. MK-801 was generalized from PCP in a dose-dependent manner, with an ED50 of 0.10 mg/kg i.p. The ED50 for PCP was 0.7 mg/kg io.p. MK-801 is, therefore, a very potent PCP-like drug which may share cellular mechanisms and other effects with PCP, including the antagonism of NMDA.