The ED50 value for exogenous gastrin stimulation of gastric acid secretion in the cat is about 300 pmol kg-1h-1. Comparison with values in man and dog show that the ED50 value for gastrin in man is approximately 6-8 times lower than in cat or dog. The rise in circulating gastrin concentration during infusion of a standard dose of gastrin was 1,340 pg ml-1 in the conscious cat. Comparable studies in man and dog give values of 900 and 300 pg ml-1, respectively. Based on these observations, the sensitivity of the parietal cells to gastrin is in the order man, 1.0; dog, 0.47; and cat, 0.09.