In eight volunteers the effect of pentagastrin (0.15, 1.0 and 6.0 microgram/kg body weight/h), secretin (0.5 and 1.0 clinical units/kg b.w./h), and cholecystokinin (CCK) (0.5 and 1.0 Ivy dog units/kg b.w./h) on the gastric secretion of pepsin was investigated to ascertain whether interaction occurred. A high intraindividual variation was found, and also a significant washout of pepsin in the initial period after stimulation. Pepsin secretion was stimulated after pentagastrin (50% above basal level) and even more after secretin (75%-200% above basal level), whereas no stimulation but a tendency for depression was seen after CCK. With the doses of gastrointestinal hormones used in this investigation, no interaction between secretin and CCK on gastric secretion of pepsin in man was demonstrated.