The Syrian golden hamster is relatively insensitive to mitotic arrest caused by colchicine when compared to other animal species. Before measuring the mitotic index in vitamin A-deficient hamsters, we determined the dose of colchicine, which arrested the maximal number of mitotic cells in metaphase. This dose of colchicine was 10 mg/kg in the esophageal epithelium, 100 mg/kg in the duodenal epithelium, and 500 mg/kg in the regenerating epithelium of the trachea. The sensitivity to colchicine was, therefore, organ dependent. The number of mitotic cells arrested by colchicine was increased in the tracheal epithelium of vitamin A- deficient hamsters. Incorporation of3H-thymidine into mucous and basal cells was also augmented and measured by autoradiography.