Postprandial plasma glucose, insulin and C-peptide profiles were studied in eight normal subjects, in the afternoon or in the evening. Two to five synchronous oscillations, with a mean period of 51 to 112 min were detected. The oscillations were highest after meals and were then damped, reverting to fasting levels after up to 340 min. Additional short-term oscillations, with periods of 20-30 min and 9-14 min, were observed. Cross-correlation studies of glucose and insulin and of insulin and C-peptide revealed a high correlation in the frequency bands considered. The synchronous oscillations of insulin and C-peptide suggest cyclic variations in pancreatic secretion rather than cyclic changes in insulin degradation.