Signal detection indices (perceptual sensitivity) were calculated to compare performance of 24 male schizophrenic inpatients and 24 controls (12 alcoholics and 12 normals) on 4 different CPT-tests. A standard version (St) employed 1 target (P = 0.166) and 5 nontargets. In condition V stimuli were presented visually, in condition A acoustically and in condition VA bimodally (1 target (P = 0.333) and 1 nontarget). Compared to controls schizophrenics exhibited lower levels of perceptual sensitivity in all 4 conditions. They were especially impaired when stimuli were presented either acoustically or when they had to monitor 2 modalities simultaneously. Perceptual sensitivity of schizophrenics was significantly lower in conditions V, A, and VA than in condition St. For controls only condition VA led to lower values. Because St was always presented first, the possible explanation that vigilance decrement over time is responsible for the lowered perceptual sensitivity had to be ruled out. It could be shown that schizophrenics did not differ in sensitivity between conditions being later in task sequence. Controls, however, showed a slight decrement over time. Thus our finding should to a large extent be attributed to different task requirements. Response criterion beta yielded inconsistent results.