We examined effects of single intake of an extract of seeds of evening primrose (EPE) on postprandial blood glucose levels on 16 healthy nondiabetic subjects (9 males and 7 females, age 32.6±10.4) and 18 subjects suffering mild diabetes and borderline diabetes (15 males and 3 females, age 53.5±7.1). After 5min of administration of EPE or placebo, the subjects were given cooked rice by crossover design. In the all subjects, the rises of postprandial blood glucose levels were reduced, by EPE ingestion compared to those by placebo ingestion. Furthermore, the rises of postprandial blood insulin levels were reduced by EPE ingestion, compared to those when placebo taking. These results suggest that the EPE may be a useful food ingredient for prevention of diabetes mellitus.