Motilin, human pancreatic polypeptide (HPP) and gastrin plasma concentrations have been determined in 196 fasting subjects ranging in age from the third to ninth decade. Detailed descriptions of the motilin, HPP and gastrin radioimmunoassays are presented. Plasma motilin concentrations ranged from undetectable to 1253 ng/L with a median value at 157 ng/L. Age and sex did not influence plasma motilin concentrations. Plasma HPP concentrations ranged from undetectable to 1564 ng/L with a median value at 142 ng/L. Plasma HPP concentrations increased significantly with age; males generally had higher concentrations than females. Plasma gastrin concentrations ranged from undetectable to 1658 ng/L with a median value of 21 ng/L. Age and sex did not influence plasma concentrations. These data establish ranges for plasma motilin, HPP and gastrin in fasting subjects aged 21--90 years using the described radioimmunoassays.