Chronic administration of a psychomotor stimulant has been shown to produce progressively enhanced effects, a phenomenon called "reverse tolerance" or sensitization. The present study reexamined the effects of the daily injection of phencyclidine on locomotor activity and stereotypy in rats, and investigated whether drug-environment conditioning was necessary for the development of behavioral sensitization and whether (5R,10S)-(+)-5-methyl-10,11-dihydro-5H-dibenzo[a,b]cyclohepten-5,1 0-imine hydrogen maleate (MK-801, dizocilpine) blocked behavioral sensitization to phencyclidine. Female Sprague-Dawley rats were used. Locomotor activity and stereotypy were measured automatically with the Digiscan system. The results confirmed an earlier finding that four daily injections of phencyclidine induced sensitization to both locomotor activity and stereotypy. The development of behavioral sensitization did not require drug-environment conditioning. Moreover, MK-801 did not block behavioral sensitization to phencyclidine. The results of the present study suggest that the neuronal mechanisms underlying sensitization to phencyclidine are different from those underlying sensitization to amphetamine and cocaine.