It is found that jactatio capitis, a stereotyped hyperkinesis in the form of pendulum-like motions of head and body, reported earlier by other authors as provoked by different neurotropic drugs, can appear in rats and mice spontaneously under emotional excitement. This hyperkinesis has different frequency in different strains, and the frequency and the degree of expression increase along with the increase of the degree of inbreeding. Hybridological analysis has shown that the jactatio capitis is connected with the suppression of eumelanin synthesis, and is a manifestation of pleiotropic effect of genotypes c or pp for C and Ploci of coat colour. It occurs in albinos and in animals of yellow colour, and is never found in those with black, brown or grey colour, or in black-, grey- or brown-spotted animals. The jactatio capitis shows a negative phenotypical correlation with emotionality as determined by the number of defecations, and has a positive connection with the predisposition to audiogenic seizures. The jactatio capitis seems not to be serotonin-ergic symptom. As the stereotypic movements are combined with a kind of "freezing", the authors consider the jactatio capitis to be a kind of catatonic syndrome.