Since metformin became available for therapeutic utilisation, more than 30 years ago, it has been found that the compound was able to reduce hyperglycaemia in diabetic subjects without any stimulation of B cell secretion. The mechanism(s) of action of this drug has been better clarified these last 5-10 years even if all its aspects are not yet fully elucidated. What has been established, however, since the beginning of its clinical use, is that metformin can act in the presence of insulin in "facilitating" its effects. This had lead some authors to investigate the possible synergistic effect of metformin added to insulin therapy. Some studies have thus shown that insulin requirements were significantly decreased during the administration of biguanides, and effect which seemed to be maximal shortly after commencing the drug. Some authors have also claimed that biguanides smooth out blood glucose profiles in brittle diabetes, but this is denied by others. A decrease in insulin requirements may be of interest in diminishing peripheral hyperinsulinism and its possible consequences. It remains questionable whether the addition of metformin in the long term is to be recommended in Type 1 diabetic patients. However, such a clarification of decrease insulin requirements can help in the understanding of the clinical significance of metformin's actions in diabetes (impact on insulin resistance, receptor and post-receptor effects).