For the elaboration of a formula estimating the reflux volume of duodenal secretion into the stomach, gastric and duodenal secretion have been studied during pentagastrin and secretin stimulation. In the stimulated gastric secretion the concentration of sodium varied with the secretion rate, whereas the concentration of chloride was nearly constant. The concentration of sodium in duodenal juice was constant, but the chloride concentration dropped significantly during secretin stimulation. Secretin induces duodenal reflux. When duodenal reflux occurs, a substantial amount of the sodium in gastric juice is attributable to intruding duodenal juice. The formula gives the duodenal reflux volume in 15-min samples on the basis of the sodium in gastric secretion and in the duodenal juice. A concentration of sodium in samples of gastric juice twice exceeding the concentration of sodium in maximally stimulated gastric secretion (8 +/- 2 mmol/l, mean +/- S.D.) may suggest 'contamination' of the samples with duodenal juice. The present formula allows for quantitative determination of this 'contamination.'