Various substrates were tested for their ability to restore the amplitude of contraction of hypodynamic electrically-stimulated rat ventricle strips suspended in a phosphate-buffered medium. Greatest recovery of the contractile activity was obtained with pyruvate; lactate, β-hydroxybutyrate and acetate were more effective than succinate and glucose. The finding that glucose, in contrast to pyruvate, was relatively ineffective as an energy source for contraction, when a phosphate-buffered medium was employed, suggests that the defect was due to a rate-limiting step in the conversion of glucose to pyruvate, rather than to an impairment in the functioning of the Krebs cycle.