SINCE an increase in cell number is one of the processes by which organs and tissues grow, it was of interest to investigate how cellular proliferation is influenced by two major growth-promoting factors, the pituitary growth hormone and the thyroid hormone, thyroxine. Replacement doses of these two hormones were therefore administered, separately or in combination, to thyroidectomized and/or hypophysectomized rats, and cellular proliferation was investigated by counting the dividing cells in the crypts of Lieberkühn of the duodenum, where under normal conditions, mitoses are abundant and appear at a constant rate (Leblond and Stevens, 1948). methods Two experiments were carried out. In the first experiment 12 groups of 10–20 male albino rats each (from the Charles River Breeding Laboratories, Wilmington, Mass.) were thyroidectomized and/or hypophysectomized when weighing 120-150 gm. and thereafter received thyroxine and/or growth hormone1 for 31 days. A thirteenth group was kept as intact control (Table 1).