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1. Pyruvate dehydrogenase complex was isolated from pigeon breast muscle involving steps of isoelectric precipitation, poly(ethyleneglycol) fractionation and separation on a glycerine gradient in the ultracentrifuge. 2. Arsenite, a potent inhibitor of the dihydrolipoyl transcetylase, did not affect the formation of acetoin from acetaldehyde, indicating that the pyruvate dehydrogenase component was operative in this reaction. 3. Production of acetoin by the pyruvate dehydrogenase complex is subject to regulation by phosphorylation and dephosphorylation, the dephosphorylated form only being active. 4. The inhibition by acetaldehyde of the pyruvate dehydrogenase complex could be partly explained by the formation of acetoin as an alternative reaction.