Systemic and pulmonary hemodynamic responses to adrenergic and cholinergic agonists were investigated in fetal lambs between 60 days and term gestation. The cardiovascular response to these agents increases with fetal age, and the increase is related to maturation of the effector system rather than the vascular receptors. The fetal pulmonary vascular bed and the ductus arteriosus are the primary components responding to acetylcholine; the systemic response is secondary to that occurring in the lung. Both fetal systemic and pulmonary vascular beds are under α-adrenergic control whereas the fetal heart is under β-adrenergic control.