Event-related brain potentials were recorded from 10 patients with DSM-IV schizophrenia (9 men) and 10 healthy control subjects (9 men) during the performance of two auditory oddball tasks, one using a 1.5 second interstimulus interval (ISI), the other using an 8 second ISI. P300 amplitude to target tones (.20 probability) and standard tones (.80 probability) were measured from midline electrodes Fz, Cz, and Pz. Results showed different effects of ISI in the two groups. Controls showed a slight decrease in P300 amplitude to targets but a marked increase in P300 to standards with the increase in ISI. In contrast, schizophrenic patients showed no change in the P300 to targets and a relatively small increase in P300 to standards with the ISI increase. Moreover, relative to the controls, P300 amplitude to targets was reduced in the schizophrenic patients at the short but not the long ISI. Implications for the cognitive significance of the P300 and its reduction in schizophrenia are discussed.