Thirteen women with late pregnancy gestational diabetes mellitus were tested with an intravenous glucose tolerance test and both blood glucose and plasma insulin levels were measured. Each woman was then treated with 100 mg. of vitamin B6 per day for 2 weeks and the intravenous glucose tolerance test was then repeated. There was a statistically significant improvement in the glucose tolerance curve after the vitamin B6 treatment, with a lowering of blood glucose levels at all points on the curve except for the 5 minute value. This glucose effect occurred despite an unchanged or lowered plasma insulin level. These results suggest that a relative deficiency in vitamin B6 is associated with some cases of gestational diabetes mellitus and that the replacement of this vitamin improves the metabolic state. The low vitamin B6 levels appear to alter metabolic pathways which result in a lowering of the biologic activity of endogenous insulin.