Blood glucose, circulating lymphocyte numbers, and the percentage of T- and B-lymphocytes were measured in diabetic and non-diabetic rats which had been fed on control diets, or diets which included oil of evening primrose or coconut oil. In diabetic animals fed on the control or coconut oil diet the number of circulating lymphocytes decreased; this was caused by a decrease in both T- and B-lymphocytes. The decreases in lymphocyte numbers was less in the diabetic animals fed on the diet enriched in evening primrose oil. In these animals the decrease in T-lymphocytes was less and the percentage of B-lymphocytes was increased. It is suggested that the diet enriched in evening primrose oil exerts its protective effect by providing gamma-linolenic acid which is necessary for biosynthesis of prostaglandin E1.