Predispositions to pendulum movements (PMs) and catalepsy was studied in rat strains selected for predisposition to catalepsy and for enhanced or suppressed PMs. Positive correlation between PMs and catalepsy at earlier stages of selection changed to negative at later stages. Based on these data, it was hypothesized that the genes that determine the predisposition to catalepsy affect the expression of the gene determining PMs in such a way that different PM alleles become dominant in the cases of moderate and strong effects of the catalepsy genes.