Visual and auditory evoked potentials were recorded from the scalp under conditions of uncertainty versus certainty, correctness versus incorrectness of pretrial guess, and shift versus repetition of the sensory modality of the stimulus. Schizophrenic patients, psychotic depressive patients, and normal controls were matched for age, sex, race, and socioeconomic status and educational level. The two patient groups were matched for medication (chlorpromazine) dosage level. Under all conditions normals exhibited the largest N1-P3 evoked potential amplitudes, while depressives exhibited the next largest and schizophrenics the smallest. All three groups had larger Ni-Pa amplitudes in the uncertain condition than in the certain condition. However, the effect of uncertainty was greatest in the normal group and least in the schizophrenic group. There were no significant interactions between group membership and guessing correctly and incorrectly. On the other hand, there was a significant interaction between group membership and modality shift. Depressives and normals tended to have larger N1-P3 amplitudes in the crossmodal condition, while schizophrenics tended to have larger amplitudes in the ipsimodal condition.