Aim: Glucose-dependent insulinotropic polypeptide-receptor (GIP-R) antagonism using (Pro(3))GIP improves glucose tolerance and ameliorates insulin resistance and abnormalities of islet structure and function in a commonly used model of obesity-diabetes, namely ob/ob mice. The effect of GIP-R antagonism in a streptozotocin (STZ)-induced model of insulin deficiency has not been evaluated. The present study has investigated the effects of daily administration of (Pro(3))GIP to STZ-treated mice. Methods: Swiss TO mice received once-daily injection of (Pro(3))GIP (25 nmol/kg body weight) or saline 4 days prior to and 16 days after injection of STZ, and effects on metabolic parameters and islet architecture were assessed. Results: (Pro(3))GIP treatment had no significant effect on hyperphagia or body weight loss. However, hyperglycaemia and glycated haemoglobin were worsened, glucose tolerance further decreased and insulin sensitivity was impaired by (Pro(3))GIP. These effects were observed on an STZ-induced background characterized by severe reductions of circulating insulin, beta-cell mass and pancreatic insulin stores. Conclusions: These data indicate that the beneficial actions of the GIP-R antagonist, (Pro(3))GIP, in obesity-diabetes appear to be largely mediated through insulin-dependent mechanisms that merit further investigation.