Alteplase is the most commonly administered thrombolytic agent in the United States. However, concurrent therapy with nitroglycerin reduces plasma alteplase concentrations and its clinical efficacy. We sought to determine if this interaction is concentration and pH dependent. Seventy plasma samples were prepared and divided into three groups: alteplase 500 IU/ml alone (group 1), alteplase 500 IU/ml plus nitroglycerin 5 ng/ml (group 2), and alteplase 500 IU/ml plus nitroglycerin 200 ng/ml (group 3). The samples were analyzed at time zero and 3 hours (incubated at 37°C). Group 1 had significantly higher plasma alteplase concentrations than group 3 (p<0.001). When alteplase and nitroglycerin are combined, the degradation of alteplase in plasma is enhanced.