OBJECTIVE Human gastric juice contains a multiplicity of proteinases, and one enzyme (pepsin 1) is increased in patients with peptic ulcer disease. However, little is known about its secretion in health. The purpose of this study was to determine the pepsin 1 content in relation to the total pepsin of human gastric juice in healthy volunteers and the effect of different stimuli on secretion. METHODS Human gastric juice from healthy volunteers was collected at 10-min intervals over a period of 30 min basally and up to 60 min after continuous intravenous pentagastrin administration (n=13) or insulin-induced hypoglycaemia (n=11). Pepsin 1 was measured by semiquantitative agar gel electrophoresis and the total pepsin as the proteolytic activity calibrated against pig pepsin A. RESULTS Pepsin 1 was present in 23 out of 24 basal gastric juice samples at a mean concentration of approximately 12 microg/ml. During continuous intravenous pentagastrin to 11 subjects for 60 min, the mean concentration and secretion rates per minute rose to 24 microg/ml and 122 microg/min at 60 min; the proportion of pepsin 1 to total pepsin was 2.8% in basal secretions increasing to 7.8% in the 60 to 70-min samples. Following intravenous insulin administration to 11 subjects, the mean concentration and secretion rates of pepsin 1 rose to 33 microg/ml and 127 microg/min at 60 min: the proportion of pepsin 1 to total pepsin was 0.8% in basal secretions and 3.1% in the 50 to 60-min samples. CONCLUSIONS Both pentagastrin and hypoglycaemia caused similar maximal secretion rates of pepsin 1, but hypoglycaemia caused greater secretion rates of total pepsin. The maximum concentrations and secretion rates of pepsin 1 occurred at different times from those of total pepsin and there is a threshold of total pepsin secretion below which pepsin 1 was not released.