p-Chlorophenylalanine is known to selectively decrease brain serotonin and to induce muricidal aggression in previously docile rats. With regard to the substance's relatively time-limited effect on brain serotonergic system, the neurochemical activity of p-CPA on tryptophan, serotonin and 5-hydroxyindoleacetic acid on seven discrete areas of the rat brain is studied in parallel to the time-course of muricidal aggression induced by the drug. Evidence indicates that, following the disruption of brain serotonergic inhibitory control, muricidal aggression becomes a stable behavior, probably on account of its self-reinforcing property.