A radiorespirometer was designed and constructed which can accommodate up to 12 tissue culture plates at one time. This apparatus was used in conjunction with specifically labelled radioactive substrates to evaluate metabolic control mechanisms in cultured neonatal rat heart cells. The basal oxidation rates of 1-(¹⁴C)-glucose, 1-(¹⁴C)- pyruvate, 1-(¹⁴C)-lactate and 1-(¹⁴C)-palmitate were 0.023, 0.144, 0.192 and 0.009 μmol/hr/mg protein, respectively. Anoxia reduced the oxidation rates of glucose, lactate and pyruvate to 0.009, 0.011 and 0.033 μmol/hr/mg protein, respectively. Halothane inhibited lactate oxidation by 64%, but did not affect the rate of palmitate oxidation. The presence of either glucose or palmitate inhibited lactate oxidation by 40-50%, and glucose inhibited pyruvate oxidation to a similar ex- tent. The rate of palmitate oxidation was not significantly altered in the presence of either glucose or lactate. Palmitate did not affect the rate of glucose oxidation. The data is discussed as it relates to elucidating the mechanisms responsible for the effects of anoxia, anesthetics and carbohydrate/lipid availability on cardiac tissue by determining the metabolic control points of heart cells.