In order to address basic mechanisms behind a reduced averaged P300 wave in schizophrenia and depression, 17 unmedicated schizophrenic and 11 unmedicated depressive subjects were tested in an 'oddball paradigm' against healthy controls matched for gender and age. The amplitude distributions of single trials' maximum positive deflections after stimulation (P300) for both target and nontarget stimuli were determined, which served as a basis for calculating the discrimination index d'. This index characterizes differences in the electrophysiological responses to target and nontarget stimuli of a subject being engaged in a discrimination task. As a main result d' was significantly lower for schizophrenics than for their controls. Directly compared to depressive subjects, schizophrenics also depicted a statistically significant decrease of the discrimination index, which could not be explained by differences in age. Although the averaged P300 signals did not show any significant differences between the two diagnostic groups, the approach of calculating d' on the basis of single trial analysis differentiated between schizophrenics and depressives. In conclusion, schizophrenic patients revealed different functional features when generating event-related potentials in an 'oddball paradigm' compared to healthy controls and depressives.