The binding properties of tetramethylpyrazine (TMPZ), a commonly used cardiovascular drug in China, to alpha 2 adrenergic receptors in human platelets were investigated. Increasing concentrations of TMPZ inhibited [3H]-yohimbine binding in intact platelets. The displacement curve was parallel to those of clonidine and yohimbine, with a Ki of 7.7 X 10(-5) M and an IC50 of 1.2 X 10(-4) M. In platelet membranes, increasing concentrations of TMPZ shifted the saturation binding curves of [3H]-yohimbine to the right and caused the dissociation constant (KD) to increase gradually. Higher concentrations of [3H]-yohimbine overcame the inhibition of TMPZ. Furthermore, in platelet cyclic AMP generation experiments, higher concentrations of TMPZ (10(-4)M-10(-6)M) inhibited the cyclic AMP increases induced by 10(-5) M PGE1. However, in the presence of an alpha 2 adrenergic receptor agonist (L-epinephrine), TMPZ blocked the inhibitory effect of L-epinephrine on cyclic AMP increases induced by PGE1. These properties suggested that TMPZ is an alpha 2 adrenergic receptor partial agonist. These effects may provide potential mechanisms for the cardiovascular actions of TMPZ.