The number of the various cell categories of the exocrine pancreas of the rat was evaluated by morphometric methods in paraffin sections of glands from rats aged 2, 5,15, 20 and 33 days. The evolution of these cell types could be properly expressed by equations of the type y = a0 e k·x, where ÿ = cell number, and x = age in days. The time necessary for each cell type to duplicate was thus obtained. The percentages (y’) of acinar, intercalated duct and connective tissue cells also proved to be age-dependent and could be expressed by second-degree equati