Behaviour was observed in mice given L-tryptophan, L-tyrosine or L-phenylalanine and then placed either in an open field or in water in a narrow cylinder. Animals placed in water (swim test) soon assume a characteristic immobile posture. Most antidepressants, including pharmacologically atypical ones, decrease immobility in the swim test while many decrease or do not affect activity in the open field test. Tryptophan increased immobility in the swim test, but did not affect open field activity and thus did not exhibit the normal profile of activity for an antidepressant. Tyrosine decreased immobility in the swim test and markedly increased activity in the open field test, an action similar to the psychostimulants amphetamine and caffeine. Phenylalanine decreased immobility in the swim test, but did not affect open field activity. Thus, its behavioural effects are similar to those of an antidepressant. As expected, tryptophan increased brain tryptophan and serotonin in a dose-related fashion. Tyrosine did not alter dopamine or noradrenaline levels, while phenylalanine lowered dopamine, noradrenaline and serotonin. These biochemical data do not fully explain the behavioural results.