The administration of p-chlorophenylalanine to mice prevents the rise in brain 5-hydroxytryptamine concentration associated with ethanol withdrawal but does not affect the increase in brain catecholamines which occurs at the same time. The locomotor excitement, piloerection, tremor and handling convulsions which occur during ethanol withdrawal were not affected. These results suggest that the increase in brain 5-hydroxytryptamine which occurs in ethanol withdrawal is a consequence of increased 5-hydroxytryptamine synthesis and that it is probably not involved in the above behavioural changes of ethanol withdrawal.