In 5 human subjects, 95% pure cholecystokinin (CCK) given as a background infusion in doses of 42, 84, or 168 pmol kg-1 h-1 did not significantly alter acid secretion in response to graded doses (11-300 pmol kg-1 h-1) of synthetic human gastrin-17-I. The 168 pmol kg-1 h-1 dose of CCK produced maximal pancreatic amylase output. In 3 subjects, 337 pmol kg-1 h-1 of CCK slightly stimulated acid secretion when given alone and tended to reduce acid secretion in response to gastrin, but each of the subjects experienced cramping abdominal pain. The increment in acid secretion produced by CCK alone was similar to that produced by maximally effective doses of carboxyl-terminal octapeptide of CCK. In dogs with gastric and pancreatic fistulas, 168 pmol kg-1 h-1 of CCK produced maximal pancreatic protein output and slightly stimulated gastric acid secretion. In dogs with gastric fistulas and Heidenhain pouches, the lowest dose of CCK that inhibited gastrin-stimulated acid secretion was 674 pmol kg-1 h-1. We conclude that in man and dog 95% pure CCK weakly stimulates gastric acid secretion and inhibits gastrin-stimulated acid secretion but these actions occur only with doses of CCK that are maximal or supramaximal for pancreatic enzyme secretion. Because of the high dose requirement, these effects are unlikely to be physiologically significant.