1 In helically-cut strips of aortae from rabbits of different ages (2 to 360 days old), responses to vasoconstrictor and vasodilator agents were compared. 2 The passive tension developed by the same magnitude of stretch was markedly less in aortae from immature rabbits than in aortae from mature rabbits. 3 Contractile responses to noradrenaline and K+ increased with age in the range from 2 to 90 days. High concentrations of noradrenaline (10(-5) and 5 X 10(-5) M) produced relaxation of aortic strips from immature rabbits (2 to 30 days). The response to histamine was not altered with age. Age-dependent reductions (90 to 360 days) in the response to 5-hydroxytryptamine were observed. 4 Relaxations induced by isoprenaline of aortic strips contracted with prostaglandin F2alpha increased with age in the range from 2 to 30 days but decreased with age from 30 to 360 days. Similar age-related alterations in the relaxant response to adenosine were observed. 5 It is concluded that aortae from immature rabbits are more distensible than those from mature rabbits; alpha-adrenoceptors mature during an early postnatal period (2 to 30 days), and 5-hydroxytryptamine receptor mechanisms deteriorate with age (older than 90 days). The beta-receptor mechanism does not seem to alter greatly with age.