In the newborn mouse the glands of Brunner consist of a few tubular downgrowths from the proximal duodenal crypts. Within three weeks the characteristic 'comma' shape of the gland is established by later downgrowths from more distal growths. The gland cells, although specialized from birth, show a high mitotic index, and a high labelling index with 3H-T, during the first three weeks, and particularly during the first two weeks. Nevertheless, the daily mitotic rate during this period is insufficient to account for the daily addition of glandular cells, suggesting that there is a continuing contribution of cells from the crypts. After this time, however, the mitotic activity of the mature gland cells is sufficient to account for the continuing increase in cell population. After the fourth postnatal week the mitotic and labelling indices decline markedly, and although mitoses are rarely seen in individual sections from older animals, cellular addition at a very slow rate is sufficient to account for the gradual expansion in size of the gland, and the process continues long after the adult body weight has been established.