Imbalances of β-adrenoceptor (β-AR) and muscarinic ACh receptor (mAChR) input are thought to underlie perinatal cardiovascular abnormalities in conditions such as sudden infant death syndrome. Administration of isoproterenol, a β1/β2-AR agonist, to neonatal rats on postnatal days (PN) 2– 5 caused downregulation of cardiac m2AChRs and a corresponding decrement in their control of adenylyl cyclase activity. Terbutaline, a β2-selective agonist that crosses the placenta and the blood-brain barrier, was also effective when given either on PN 2– 5 or during gestational days 17– 20. Terbutaline failed to downregulate brain m2AChRs, even though it downregulated β-ARs; β-ARs and m2AChRs are located on different cell populations in the brain, but they are on the same cells in the heart. Destruction of catecholaminergic neurons with neonatal 6-hydroxydopamine upregulated cardiac but not brain m2AChRs. These results suggest that perinatal β-AR stimulation shifts cardiac receptor production away from the generation of m2AChRs so that the development of sympathetic innervation acts as a negative modulator of cholinergic function. Accordingly, tocolytic therapy with β-AR agonists may compromise the perinatal balance of adrenergic and cholinergic inputs.