This study examined the effects of phenothiazine treatment on attentional-perceptual, cognitive, and psychophysiological dysfunction in chronic schizophrenics. Under double-blind conditions, 20 patients receiving chlorpromazine and 20 receiving placebo for eight weeks were tested by performance measures, clinically rated, and monitored for skin resistance and heart rate on four occasions. Phenothiazine effects on measures of attention-perception and on psychophysiological response were demonstrable, but not on tests and ratings of cognitive dysfunction. The direction of effects was toward normalization of function. Drug treatment tended to improve ability to sustain set, to increase efficiency of selective attention, and to increase rate of information processing. Autonomic reactivity was reduced and a deactivation effect suggested. Clinical improvement was correlated with reduction in attentional dysfunction. The results are discussed in terms of their implications for hypothesized behavioral mechanisms of drug action, primary "behavioral site" of drug action, therapeutic response measurement, and functional theories of schizophrenic psychopathology.