4-Hydroxytryptamine creatinine sulfate (intravenously, tail vein; administered with a constant amount of [ 3 H] norepinephrine; 17-20 g male albino mice) inhibits [ 3 H] norepinephrine uptake with an ED 50 of 35 μmoles/kg[1]. 4-Hydroxytryptamine creatinine sulfate (subcutaneous injection; pretreatment [ 3 H] norepinephrine is administered intravenously 60 min) increases the ED 50 value (21 μmoles/kg) for the release of [ 3 H] norepinephrine in monoamine oxidase-inhibited mice[1].