Age-related changes in the sensitivity to verapamil and sodium nitroprusside were examined in isolated aortic strips of the rabbit. In the aortae of newborn rabbits within 10 days of birth, the resting tone of the muscle was strongly reduced by sodium nitroprusside but not by either Ca-deficient solution or by verapamil. High K-induced contraction and noradrenaline-induced contraction were both inhibited by verapamil or sodium nitroprusside. In the aortae of 24 day-old rabbits, resting tension was slightly reduced by sodium nitroprusside but not by verapamil. High K-induced contraction was less sensitive to sodium nitroprusside than to verapamil whereas noradrenaline-induced contraction was less sensitive to verapamil than to sodium nitroprusside. In the aortae isolated from 60 day-old or older rabbits, resting tension was not affected by either sodium nitroprusside or verapamil. High K-induced contraction was inhibited by verapamil whereas sodium nitroprusside showed only a weak inhibitory effect. Noradrenaline-induced contraction was inhibited by sodium nitroprusside although verapamil had only a slight inhibitory effect. In the aortae of 1 day-old and also in adult rabbits, noradrenaline induced an additional increase in muscle tension when applied during the sustained contraction induced by high K. It is suggested that, in the newborn rabbit aorta, the voltage-dependent Ca channel is sensitive to both verapamil and sodium nitroprusside and the sensitivity to sodium nitroprusside gradually decreases during maturation whereas the receptor-linked Ca channel is also sensitive to both of the inhibitors at birth but the sensitivity to verapamil gradually decreases with age.