The effects of testosterone on paw preference assessed by a food reaching test was studied in adult cats. It was found that tetosterone propionate injected intramuscularly reduced the degree of the right-paw preference in right-preferent female cats. The paw preference of the male cats and left-preferent female cats was not affected by this male sex hormone. It was concluded that testosterone may especially suppress the left brain to induce ambilaterality in right-paw preference in adult female cats.