Differences in the vascular alpha 1-adrenoceptor reserve and alpha 1a to alpha 1b subtype ratio between old (18 months) and young (3 months) rats were investigated by both in vitro and in vivo experiments. For the in vitro experiments, ring preparations of blood vessels with endothelium removed were prepared and were perfused with Kreb's solution. Antagonists for alpha 2- and beta-adrenoceptors were put in perfusate in order that norepinephrine (NE) could only activate alpha 1-receptor. The results showed that, in comparison with young rats, the maximal contractions induced by NE in aorta, renal arteries and mesenteric arteries isolated from old rats were not changed but the concentration-response curves were shifted to the right. Functional dissociation constant KA for NE was not changed but the ratio of KA to EC50 was decreased. In the aortae and mesenteric arteries from old rats, the blocking effect of chlorethylclonidine (CEC, selective alpha 1b-subtype antagonist) on NE-induced contraction was decreased significantly, while the effect of nifedipine (selectively blocks the response of alpha 1a-subtype) was increased. The in vivo experiments showed that the effect of nifedipine in lowering blood pressure was stronger in the old rats than in the young rats, while following administration of nifedipine, the effect of phenylephrine in raising blood pressure was weaker. These results indicate that, in comparison with the young rats, the alpha 1-adrenoceptor reserve is decreased and the ratio of alpha 1a-subtype to alpha 1b subtype is increased in blood vessels of old rats.