It has been suggested that biguanides should be used in Type 1 (insulin-dependent) diabetic patients in order to diminish insulin requirements and reduce the chances of insulin reactions. The efficacy of these compounds in such patients has been controversial. We have studied the effect of metformin (850mg) given at 08.00 h in diminishing insulin needs after a 60g carbohydrate mixed meal taken at 12.00 h, using an artificial pancreas and a sequential analysis of the results. The morning test dose of metformin or placebo was preceded by 48 h treatment with metformin (850mg twice daily) or placebo. After the eighth patient a 26% saving of insulin need was demonstrated in the metformin-treated group (p<0.01). Metformin is thus effective in reducing post-prandial insulin needs in Type 1 diabetic patients, although its use in such circumstances requires consideration of several other issues.