Endothelium-dependent relaxations mediated by Gi protein are prominently impaired in atherosclerotic coronary arteries. However, it remains to be determined whether the expression of endothelial Gi protein per se is reduced in coronary atherosclerosis. Thus, in the present study the expression of endothelial Gi protein was examined by immunohistochemical staining using a specific antibody against human Gi protein (alpha-subunits of Gi-1 and Gi-2 proteins) in the proximal segment of the left anterior descending coronary arteries (segment 6) from 40 consecutive autopsy cases. The immunoreactive level of the Gi protein was semi-quantitated in four grades (none, 0; slight, +; moderate, +2; high, +3) and the mean value of the ratings of all endothelial cells was used as an index of the endothelial Gi protein expression of the artery. The immunoreactive level of the Gi protein in human coronary arteries was significantly reduced with ageing and extent of coronary atherosclerosis (both P < 0.05), and was lower in patients with than in those without hypertension (P < 0.01) or hyperlipidaemia (P < 0.05). In addition, the level was significantly lower in the eccentric portions than in the concentric ones in each atherosclerotic coronary artery (P < 0.0001). These alterations in the immunoreactive level of endothelial Gi protein in human coronary arteries may explain, in part, why Gi protein-mediated, endothelium-dependent relaxations are prominently impaired in atherosclerosis.