This study aimed to (i) determine the changes in evoked potentials (EPs) associated with hospital treatment involving either antipsychotic (AP) drugs, tricyclic (TTR) drugs, or no medication (NM); (ii) determine which EP changes may be direct drug effects; (iii) evaluate effects of testing patients receiving AP and TR drugs on psychiatric correlates of EPs. Somatosensory (SEP) to left and right median nerve stimuli, visual (VEP), and auditory (AEP) EPs were recorded from one eye and 14 scalp leads. Patients were tested before and during AP (N = 28), TR (N = 10), or NM (N = 15) treatments; results of 35 age- and sex-matched nonpatients were compared with predrug and drug test results of 35 patients. Main findings included (i) with AP and TR drugs, SEP peak P30 amplitude tended to decrease and SEP N130 to increase; (ii) with AP drugs and NM, VEP amplitudes decreased; (iii) AEP amplitudes increased with AP drugs; (iv) changes in amplitude measures did not differ significantly between AP, TR, and NM treatments; (v) marked later SEP amplitude differences between patients and nonpatients were about the same for pretreatment and drug tests; (vi) patient-control differences were greater with drugs for VEPs and less with drugs for AEPs. It was concluded that later SEP amplitude correlates of psychopathology are little influenced by AP and TR drugs, but that VEP and AEP results are modified.