Injections of the serotonin precursor l-tryptophan (25, 50, and 100 mg/kg IP), inhibited mouse killing behavior in rats, as indicated by a dose dependent increase in latencies to attack and kill mice. Tests in 24 hr food deprived rats revealed that feeding behavior was also significantly decreased by about 30% by tryptophan injections (50--100 mg/kg IP). Concomitant with the behavioral changes were increased levels of brain serotonin and its metabolite 5-hydroxyindoleacetic acid. Drinking, latencies to sniff mice, and ability to locomote on a rotating rod were not affected by l-tryptophan injections, although spontaneous activity in an open field was reliably reduced by 33% with a dose of 100 mg/kg. Thus, while the degree of selectivity for tryptophan's effects on behavior remains open to question, these findings are consistent with hypotheses of an inhibitory role for central serotonergic systems, particularly in mouse killing and feeding behaviors.