The CCK- and secretin stimulated pancreatic volume, bicarbonate and enzyme secretion was investigated before and during fasting for 20 days in 12 obese subjects. Pancreatic function tests were performed at the start of the fasting period and on the 10th and 20th day. In an additional and comparable group of 8 obese patients plasma concentrations of cholecystokinin (CCK), gastrin and insulin as well as metabolic parameters (bicarbonate, beta-OH-butyrate and free fatty acids) have been measured before and during total fasting. During three weeks of total fasting the average overweight of the patients undergoing pancreatic function tests was reduced from 49 to 31% (-11.5 +/- 1,5 kg). A significant reduction of volume, bicarbonate, trypsin and amylase secretion occurred already after 10 days of total fasting. After 20 days these parameters were further significantly reduced compared to the 10th day. All mentioned parameters were found at the lower limit of the normal range at the end of the fasting period at 20 days. The only exception being lipase secretion; the decrease of this enzyme was not significant at the 5% level. No significant reduction of basal plasma concentrations could be observed for CCK and gastrin during the course of total fasting. The plasma insulin levels were significantly reduced after 7 days whereas at 10 and 20 days insulin concentration was not significantly lowered compared to day 1. A small but significant decrease in the blood bicarbonate concentration could be observed after 7 days of fasting which remained constant up to the end of the study.(ABSTRACT TRUNCATED AT 250 WORDS)