We investigated the antiplatelet effect of a newly synthesized guanidine derivative KR-32558, a sodium-hydrogen exchanger-1 (NHE-1) inhibitor, together with the elucidation of the possible mechanisms of action. KR-32558 concentration -dependently inhibited the aggregation of washed rabbit platelets induced by collagen (10 ${mu}g/ml$) with an $IC_{50}$ value of 85.9 ${mu}M$, but with much weaker potency against aggregation induced by thapsigargin (0.5 ${mu}M$) or A23187 (5 ${mu}M$). And had no effect on platelet aggregation induced by arachidonic acid (100 ${mu}M$), thrombin (0.05 U/ml) and U46619 (1 ${mu}M$) up to 100 ${mu}M$. KR-32558 completely inhibited the $[Ca^{2+}]_i$ mobilization induced by collagen at concentration of 100${mu}iM$. Taken together, these observation suggest that KR-32558 selectively inhibited collagen-mediated platelet aggregation by blocking the cytoplasmic calcium mobilization in addition to NHE-1 inhibition.