Both p-chlorophenylalanine (PCPA) and PCPA methyl ester were found to reliably induce mouse-killing in non-killer rats only when unusually large doses were used (three successive daily injections of 300 mg/kg) and brain serotonin (5-HT) concentration was drastically reduced (about 90 percent). Neither three doses of 100 mg/kg of PCPA nor p-chloroamphetamine (3 times 3.5 mg/kg) caused similar effects in spite of the fact that these compounds depleted brain 5-HT by 85 percent and 60 percent, respectively. PCPA-induced mouse killing was reversed by 5-HTP (100 mg/kg) only when this serotonin precursor completely restored levels of 5-HT. The topography of PCPA-induced killing did not resemble normal interspecies aggression and was also directed toward fat pups. These findings suggest that 5-HT depletion might facilitate nonspecific killing reactions, but is not a sufficient condition to induce the species-specific predatory behavior in rats.