SummaryThe fetal sheep heart responds to &bgr;‐adrenergic stimuli; however,in vivostudies show the response of the fetal heart is less than that of the adult heart. We used [3H]dihydroalprenolol (DHA) to study directly &bgr;‐adrenergic receptors in heart particulates of fetal sheep at term and adult sheep. [3H]DHA binding to fetal heart particulates was rapid, reversible (t1/2= 2.9 ± 0.3 min), stereoselective, saturable (101.2 ± 7.4 fmoles/mg protein), and of high affinity (4.8 ± 0.4 nM). The rank order of agonists competing for [3H]DHA binding was isoproterenol (0.32 ± 0.10 &mgr;M) > epinephrine (1.19 ± 0.23 &mgr;M) ⋍ norepinephrine (2.67 ± 0.69 &mgr;M), which is compatible with &bgr;1‐adrenergic potencies. [3H]DHA also bound to the adult sheep heart in a manner expected for &bgr;1‐receptors. No difference in the binding affinity of [3H]DHA or agonists' competition was demonstrated between the fetal and adult sheep heart. Comparison of the concentration of &bgr;‐adrenergic receptors in fetal and adult hearts was confounded by the choice of the denominator for unit expression. The concentration was higher in the adult when expressed as a function of protein content or 5′‐nucleotidase activity (0.52 ± 0.07versus1.12 ± 0.06). However, there was no difference when tissue weight, Na+− K+‐ATPase, or NaF‐stimulated adenylate cyclase was used. Furthermore, isoproterenol‐stimulated adenylate cyclase and cardiac contractile response to a threshold dose of isoproterenol were identical in the fetal and adult sheep heart. We conclude that &bgr;‐receptors can be studied with [3H]DHA in the fetal sheep heart, this receptor is qualitatively similar to the &bgr;‐receptor in the adult sheep heart, and it is unlikely that there is a difference in the concentration of &bgr;‐adrenergic receptors in fetal and adult sheep heart.SpeculationThe interactions of adrenergic agents with cardiac &bgr;‐adrenergic receptors in the term fetus are identical to those in the adult, and the receptor‐adenylate cyclase coupling system reaches maturity before birth.