This article reviews the effects of pregnancy on carbohydrate metabolism and insulin production in the normal rat and discusses some animal models of potential value for the study of gestational diabetes mellitus (GDM). Against the background of current clinical and laboratory experiences it is suggested that GDM reflects a deficiency in islet B-cell proliferation in response to the increased insulin requirement during pregnancy. Although this hypothesis lends itself for testing in animal experiments, a suitable animal model for GDM needs to be described.