Hydroquinone and (-)-epicatechin were isolated from seeds of Aesculus californica NUTT. (Hippocastanaceae), and their effect on the growth of lettuce and rice seedlings was examined. Hydroquinone inhibited germination at high concentrations (above 100μg/ml) but it stimulated growth at low concentrations for both lettuce and rice seedlings. (-)-Epicatechin showed little activity by itself. However, when applied with hydroquinone, (-)-epicatechin tended to counteract the growth inhibitory activity of hydroquinone.