Platelet monoamine oxidase (MAO) activity in seven chronic schizophrenic patients undergoing hemodialysis treatment was studied. The treatment was performed in a double-blind crossover design. A significant increase in MAO activity was observed after 5 h of active dialysis, whereas inactive dialysis did not significantly alter the enzyme activity. The increase in MAO activity could be explained by the observed significant decrease in apparent Km values for the amine (phenethylamine) studied. During a treatment period of 8 weeks, the MAO activity was not significantly altered with either active or sham dialysis. No relation between clinical ratings and platelet MAO activity was observed. Plasma from the seven patients was collected before and after 5 h of active or inactive dialysis and incubated with rat brain mitochondrial MAO (RBM-MAO). No difference was noted in the RBM-MAO-activity-regulating properties of the plasma sampled during either sham or active dialysis.