The influence of phenothiazine medication on auditory attention in normal and paranoid and nonparanoid schizophrenic subjects was studied. Nonparanoid schizophrenics off medication made more signal detection errors than either paranoid schizophrenics (off or on medication) or normals. Paranoid and normal subjects did not differ from each other. On medication, nonparanoids performed more like normals. The positive influence of medication is attributed not to a “normalizing” effect but mostly to an interference with basic psychopathology, primarily by depressing available psychic energy and decreasing the ability of patients to give disproportionate attention to irrelevant information. By comparing the results in this and related studies, it appears that phenothiazine medication is associated with improved signal detection performance in nonparanoid schizophrenics, with no effect in paranoid schizophrenics and with impaired performance in normals. With or without medication no differences in auditory threshold were found between normal, paranoid and nonparanoid subjects even when threshold differences were measured to within 0.25 db. Omission and commission errors were found to be related primarily to task characteristics and patient IQ. Patients with IQs of 100 or more made more omission errors than those with lower IQs. Interpretations of these and other findings are presented as well as suggestions for future research.