Effects of seven limonoids, obakunone (1), 7α-obakunol (2), 7β-obakunol (3), limonin (4), 7α-limonol (5), 7β-limonol (6) and nomilin (7), on the sleeping time induced in mice by anesthetics were assayed.All the limonoids, except for 2, shortened the sleeping time induced by α-chloralose and urethane. 7 gave the highest reduction rate of sleeping time, and the order of the reduction rate of sleeping time was as follows; 7>1 and 3>4, 5 and 6.As the chemical structures of these compounds are similar to each other, the relationship between the structure and the effects of limonoids on sleeping time was discussed.