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Summary: The ability of the heart to increase in size when subjected to an increased work load, a process called compensatory hypertrophy, is dependent upon the availability of cofactors, which, if deficient, may limit the extent of activation of protein and nucleic acid synthesis. We report here some results of the effects of orotic acid or uridine, administered to rats by oral intubation (10 mg/kg/day) and supplemented with vitamin B12 (100, μg/kg/day) and folic acid (2 mg/kg/day), as agents which enhance biochemical and physical parameters during myocardial hypertrophy. Four days after the onset of hypertrophy, the orotic acid and cofactor treated rats showed enhanced myocardial contractility as measured by isometric contractility studies using an isolated papillary muscle preparation. This treatment also enhanced the rate of total myocardial protein synthesis giving an earlier and higher peak in this parameter after the onset of hypertrophy. It has also been shown that orotic acid and cofactor treatment does not cause an elevation in the activity of circulating enzymes, nor any ultrastructural alterations in the heart. The studies reported here support the claim that orotic acid and cofactor administration enhance the processes of adaptation of the myocardium to increased work load.