The rate of salvage of purine nucleotides from hypoxanthine in glycolyzing, cultured rat heart cells was found to be decreased when respiration was suppressed. Pyruvate or phenazine methosulfate, acting as hydrogen acceptors, reversed the effect of the respiratory block. The inhibition and the reversal could not be attributed to the limitation of energy supply or of 5-phosphoribosyl-1-pyrophosphate. A causal connection was, however, shown to exist between this inhibition and the concomitant shift in the redox state of NAD+ in favor of NADH. NADH also inhibited the key enzyme of the salvage pathway, hypoxanthine-guanine-phosphoribosyltransferase, in cell-free extracts. Regulation of purine nucleotide synthesis by the redox state of NAD+ in heart cells might gain significance during transition from respiring to hypoxic state and vice versa.