The amino acid tryptophan is a precursor for serotonin, a neurotransmitter involved in various behaviours. Mal-absorption of tryptophan may thus lead to deficits in serotonin-related behaviour. Vitamin B3 is thought to have a beneficial effect on tryptophan mal-absorption, resulting in improved behaviour. The aim of this study is to test this hypothesis. In a double blind time series experiment two subjects and two controls were monitored for eight weeks. After initial periods of various lenghts to establish a baseline of the behavioural parameters, the experimentals received vitamin B3 for three weeks. In the remaining five weeks they received a placebo. The controls did not receive either placebo or vitamin B3. Four behavioural parameters (aggression, automutilation, contact and hyperactivity) were scored twice a day by members of the institution. The results do not support the hypothesis that vitamin B3 has a beneficial effect on behavioural deficits.