The action of synthetic and natural secretin on the human exocrine pancreas was compared. The trial was an intrasubject crossover comparison with randomized allocation of compounds in 12 healthy male subjects. No significant difference between compounds was found in bicarbonate, amylase and trypsin secretion after bolus injection. However, synthetic secretin evoked a significantly greater volume and amount of bicarbonate after infusion than the natural compound whilst no difference resulted in the other variables. It is concluded that synthetic secretin is suitable for testing the function of the exocrine pancreas.