Eleven patients with ischemia heart disease (IHD) were treated with low dose aspirin (ASA, 50mg/day) for more than two weeks (ASA group). 29 cases with IHD not taking ASA served as patient control (NASA group) and 13 cases without IHD not taking ASA as normal control. Blood samples for measurement of plasma (serum) TXB2 and 6-keto-PGF1 alpha were simultaneously taken from aortic root (AO) and coronary sinus (CS). The results showed: ASA group had lower plasma TXB2 level in AO blood than NASA group (P < 0.05), but there was no significant difference in plasma 6-keto-PGF1 alpha level between the two groups. Both of plasma and serum TXB2/6-keto-PGF1 alpha ratios in AO blood in ASA group were significantly lower than those in NASA group (P < 0.05 and P < 0.0005 respectively). Plasma TXB2 CS/AO ratio and 6-keto-PGF1 alpha CS/AO ratio in ASA group were significantly lower than those in NASA group (P < 0.05), but not different from those in control group. Both ASA and NASA groups had lower serum TXB2 CS/AO ratios than control group (P < 0.05). The results suggest that low dose aspirin inhibits selectively TXA2 synthesis in systemic circulation and inhibits synthesis and/or release of TXA2 and PGI2 equally (no selectivity) in coronary circulation, but could not completely inhibit intracoronary platelet activation.