The investigations of the EEG during an open study with the antipsychotic drug fluperlapine in acute schizophrenic patients are reported. Due to ethical and practical considerations some of the common pharmacoelectroencephalographic procedures as well as placebo controlled study designs could not be applied in these patients. To overcome at least partly these limitations, intraindividual as well as interindividual correlations were used. They were computed between plasma concentrations of unchanged fluperlapine as well as its metabolite N-oxide fluperlapine, on one hand, and EEG variables, on the other. The intraindividual correlations can be computed either on the first day of the active treatment over various time points of that day (acute effects) or across several appointed treatment days always taking values at the same time during these days (chronic effects). The intraindividual correlations of a set of subjects were submitted to a sign test to obtain an overall result for the relation between the EEG and blood plasma levels of the drug. In this way an acute and a chronic effect of fluperlapine on the EEG could be shown consisting mainly of an increase in slow waves, a decrease in the α-activity and a tendency of β-activity to decrease. A comparison of the correlations between the plasma levels of fluperlapine and the EEG variables with the correlations between the plasma levels of N-oxide fluperlapine and the EEG give rise to the hypothesis that unchanged fluperlapine has a stronger effect on the EEG than its metabolit