Lysophosphatidylcholine (LPC), which accumulates in atherosclerotic arteries, has been reported to inhibit endothelium-dependent relaxation (EDR) in many different species. However, the underlying mechanism of LPC-induced inhibition of EDR is still uncertain. In the present study, we measured simultaneously both isometric tension and cytosolic free Ca2⁢ ([Ca2⁢]i) in rabbit carotid strips, and examined the effect of LPC on tension and [Ca2⁢]i. In carotid strips with intact-endothelium, high K⁢ (70 mM) increased both tension and [Ca2⁢]i, and cumulative addition of acetylcholine (ACh) from 0.1 to 10μM induced dose dependent increase of [Ca2⁢]i with concomitant relaxation. In the presence of L-NAME (0.1 mM), ACh increased [Ca2⁢]i without affecting the amplitude of high K⁢-induced tension. These ACh-induced change of [Ca2⁢]i and tension was abolished by removal of endothelium or 10 nM 4-DAMP (muscarinic receptor antagonist) pretreatment. Pretreatment of LPC (10μM) inhibited ACh (10μM)-induced change of tension and [Ca2⁢]i in endothelium-intact carotid artery. On the other hand, LPC had no effect on ACh-induced change of tension and [Ca2⁢]i in endothelium denuded artery. In Ca2⁢-free external solution, ACh transiently increased [Ca2⁢]i, and pretreatment of LPC significantly inhibited ACh-induced transient [Ca2⁢]i change. Based on the above results, it may be concluded that LPC inhibits the ACh-induced [Ca2⁢]i change through inhibition of Ca2⁢ mobilization in vascular endothelial cells, resulting in decreased production of NO and concomitant inhibition of endothelium- dependent vascular relaxation.