The release of immunoreactive gastric inhibitory polypeptide (IR-GIP) in response to a standard meal was examined in 10 normal subjects and 15 type I (insulin-dependent) diabetics 7 days (test I), 14 days (test II), and 3 months (test III) after time of diagnosis. During all three tests, the diabetics had significantly lower plasma IR-GIP concentrations than the controls from 15-90 min after the standard meal. The IR-GIP response in the diabetics measured as the integrated area under the response curve corresponded to 70% of that of normal subjects. beta-cell function evaluated from the C-peptide response to the meal increased significantly from test I to test III whereas the IR-GIP response was similar during all three tests. As GIP is known to potentiate glucose-induced insulin secretion and possibly the biosynthesis of insulin, the low IR-GIP responses in subjects with type I diabetes may significantly influence insulin levels and hyperglycemia.