The pig heart grows at a maximal rate in the first 2-3 days of life due to a volume overload imposed on the heart at birth. Rates of ribosome formation and protein synthesis cannot be further accelerated during in vitro perfusion with agents that increase cyclic AMP, that bind to alpha 1-adrenergic receptors or that bind to angiotensin II receptors. Growth of the heart in vivo can be restrained by treatment with an angiotensin-converting enzyme inhibitor, enalapril maleate, or an angiotensin receptor antagonist, DuP 753. In the enalapril-treated heart, norepinephrine plus propranolol, but not angiotensin II, accelerated ribosome formation. Rapid growth of the left ventricle of pig heart during the first 10 days of life is due largely to eccentric hypertrophy.