The effects of dietary calcium, buffering capacity (BC), lactose and ethylenediaminetetraacetic acid (EDTA) on the pH pattern of and calcium absorption from 6 segments of the gastrointestinal tract were studied. One hundred and forty-four rats were used for a replicated, factorially arranged experiment, which involved calcium (0.19, 0.48 and 0.78%), BC 1) 0.46 ml, 2) 0.54 ml and 3) 0.76 ml N HCl required to reduce the pH of 5.0 g of the diet by one pH unit), lactose (0 and 12%), and EDTA (0 and 0.2%). Chromic oxide served as a reference substance for calcium absorption. The increased calcium absorption with decreased calcium intake was a result of increased hydrogen ion concentration in the intestinal secretion; decreased endogenous secretion of calcium in the duodenum which increased dietary calcium absorption in the lower gastrointestinal tract; and increased calcium absorption all along the intestinal tract posterior to the duodenum. BC controlled the pH of gastrointestinal contents and lactose produced a more acid pH in the ileum. The percentage calcium absorption increased with the more intestinal acid pH, and decreased with dietary increases in calcium, BC or EDTA. Calcium and BC of the diet interacted and influenced calcium secretion in the duodenum and calcium absorption in the jejunum and ileum.