Clonidine causes dilatation of the aorta in the presence of endothelium, while it causes contraction of the aorta in the absence of endothelium. The present study was carried out to clarify the role of alpha-1-adrenoceptors in the vascular action of clonidine. The aortic rings were suspended in Krebs-Henseleit (K-H) medium, and the effects of alpha-1- and alpha-2-adrenoceptor antagonists on the clonidine-induced contractions were measured. Moreover, the role of the phosphatidylinositol (PI) response was examined. The aortic slices were incubated in K-H medium containing, [3H]myo-inositol and clonidine. The formation of [3H]inositol monophosphate (IP1) was measured with a liquid scintillation counter. Clonidine caused contraction of the aorta in the absence of endothelium, in a dose-dependent manner. This contraction was inhibited by antagonists in the following order of the potency: prazosin > phentolamine > spiperone > urapidil = yohimbine > L-659066 > atipamezole. On the other hand, clonidine inhibited norepinephrine (NE)-induced contraction in the aorta in the absence and in the presence of endothelium. Clonidine enhanced IP1 accumulation in the aorta in the absence of endothelium, whereas it inhibited NE-induced IP1 accumulation in the aorta. The present results show that alpha-1-adrenoceptors are probably involved in the clonidine-induced contraction and relaxation of the rat aorta.