Responses to vasoactive agents were compared in helical strips of basilar arteries isolated from rabbits, 15 to 360 days of age, which were stretched under optimal resting forces. Contractions induced by serotonin, histamine, norepinephrine, angiotensin II and K+ increased with increasing age. However, the maximum forces developed by these drugs relative to those by 50 mM K+ as well as ED50 values of these drugs were not significantly different in the arteries from mature and immature rabbits; only the response to serotinin at day 360 was significantly less. Basilar arteries contracted with histamine relaxed in response to isoproterenol; the relaxing effect was suppressed by treatment with 10-6 M propranolol and was inversely related to age. On the other hand, relaxations induced by adenosine and prostaglandin E1 did not significantly differ in the arteries from rabbits of different ages. Relaxations induced by small amounts of K+ (0.5-10 mM) increased with age; the relaxations were suppressed by ouabain. It may be concluded that the cerebroarterial relaxation related specifically to the beta adrenergic mechanism is reduced with increasing age. The electrogenic Na+ pump mechanism appears to mature in rabbit basilar arteries during an early postnatal period.