The stereotyped hyperkinesis referred to as pendulum movements (PM) may be found in up to 50% of the animals in stocks of Wistar rats. The mode of inheritance of predisposition to PM was studied by two methods: 1) a classical Mendelian analysis of hybrids of the strains PM+ and PM- bred from Wistar stock for enhancement and absence of PM, respectively, and 2) a segregation analysis of pedigree data from the archive records of breeding the cataleptic GC strain. The two methods gave the same result: the inheritance of predisposition to PM can be explained by a major gene model with an incomplete penetrance of heterozygous genotype.