The effects of size, time of day and sequence of meal ingestion were determined in healthy subjects using a Latin square design. Plasma glucose, insulin and gastric inhibitory polypeptide, but not glucagon, were correlated with meal size. Plasma glucose, but not insulin, gastric inhibitory polypeptide or glucagon, were greater later in the day. The progressive decline in carbohydrate tolerance from 08.00 to 18.00 h was associated with impaired insulin secretion estimated by C-peptide, and with impaired insulin action.