Six normal subjects were tested with various doses of histamine acid phosphate (1, 10, 40, and 80 mug/kg/hr) and pentagastrin (0.15, 1.5, 6.0, and 12.0 mug/kg/hr), the different doses of the stimulants being administered by separate continuous intravenous infusions. In each sample of gastric juice, which was collected at 15-minute intervals, we estimated the concentrations of the H(+), Na(+), K(+), and Cl(-) and the concentration of pepsin. The drugs elicited equal maximal outputs of acid and pepsin. Pentagastrin was more potent than histamine in stimulating acid and pepsin secretion, and the rate of the responses was faster with pentagastrin than with histamine. Apart from this, however, the patterns of secretion of the various constituents of the gastric juice and the interrelationships between the concentrations of the electrolytes were identical with the two drugs. We therefore concluded that the actions of histamine acid phosphate and pentagastrin on human gastric secretion were identical.