Responses of isolated helical strips of ovine ear artery to electrical stimulation of postganglionic adrenergic neurons and exogenous agonists were studied at various stages of development from 110 days of gestation through to adulthood. Only rudimentary responses were observed at 110-115 days of gestation. A parallel development of responses to noradrenaline (NA), serotonin, and lysine vasopressin began sometime after 110-115 days of gestation and continued until 133-137 days of gestation but there was little development of the latter responses until more than 3-5 days post partum. Development of responses to exogenous agonists was incomplete 2-3 weeks post partum. The development of postganglionic adrenergic responses lagged behind those to exogenous NA. Two to three weeks post partum the NA maximal response was one-third that of adult tissue whereas the response to 16 Hz (highest frequency used) was one-sixth that of adult tissue. The NA threshold concentration was lower in arterial strips of adult animals than it was in those of younger animals. The data suggest that development of functional post-ganglionic adrenergic innervation of vascular smooth muscle begins late in gestation and continues well after birth; this development is preceded by development of vascular mechanisms involved in the response to several agonists.