Tryptophan restriction in the diet leads to low levels of brain serotonin. Serotonin has been implicated in the magnitude of novelty-induced grooming behavior, but its possible role in self-grooming chain completion has not been investigated. A tryptophan-deficient diet produced fewer chain-associated face washings, more face washings alone, fewer number of chains, as well as elementary units into chains. Thereafter, an apparent lower threshold for emotional responsiveness also took place. Impairments of the serotoninergic inhibitory activity of striatal and/or nigral dopaminergic terminals is suggested, because corpus striatum has been proposed to be an organizer structure of both the serial ordering as well as in the completion of the self-grooming chains.