Clinical observations indicate that atherosclerotic vessels are prone to develop vasospasm. It was assumed that this property is related to hypercholesterolemia, but the basic mechanisms are still unknown. To investigate further mechanical stimulation experiments were performed with segments of rabbit carotid arteries. Arteriosclerotic lesions were induced in vivo in these arteries by application of DC impulses either in hyper- or normo- cholesterolemic animals. Contractions were evoked by noradrenaline, KCl or hydrogen peroxide. The results show that all the arteriosclerotic segments were hypersensitive to noradrenaline and hydrogen peroxide, compared with the corresponding controls. An increase in contraction force was also found upon application of KCl to lipid-containing plaques. However, in arteriosclerotic segments obtained from normally fed animals, the contraction response to KCl was lower than in controls. This indicates that complex alterations in the contractile properties occur in smooth muscle during atherogenesis which cannot be explained solely on the basis of the influence of cholesterol on the arterial cells.