Some pharmacological actions of intraperitoneally (i.p.) administered d-pseudoephedrine (d-pseudo) on the central nervous system of mice were compared with those of l-ephedrine (l-eph). Locomotor activity was increased by l-eph (50, 100 and 200 mg/kg,) but not by d-pseudo (50, 100 and 200 mg/kg). Wheel-revolving activity was reduced by a 200 mg/kg dose of both drugs. Rectal temperature was increased by l-eph at 50, 100 and 200 mg/kg. However, it was decreased by 50 and 100 mg/kg doses of d-pseudo, and the change in response to 200 mg/kg of d-pseudo was biphasic, decreasing first and increasing subsequently. Pentobarbital-induced sleeping time was shortened by l-eph (50 and 100 mg/kg), but was not influenced by any of the doses of d-pseudo. In the experiment on locomotor activity, L-3,4-dihydroxyphenylalanine (400 mg/kg, i.p.) potentiated the effects of d-pseudo and l-eph, but reserpine (1 mg/kg, day X 5 days, i.p.) and 6-hydroxydopamine (40 micrograms/mouse, intracerebroventricular injection) potentiated only the effect of l-eph. These results suggest that the effect of d-pseudo on the central nervous system of mouse is not only weaker than that of l-eph, but also the mechanisms of action of these two substances are different.