1. The pyruvate dehydrogenase complex from human heart has been partially purified and shown to be regulated by a phosphorylation-dephosphorylation cycle similar to that previously found for other mammalian tissues. 2. Incubation of the complex with ATP (2 mmol/l) led to its inactivation associated with the concomitant incorporation into the protein of 32P from the terminal phosphate group of the ATP. Pyruvate, ADP, thiamin pyrophosphate and dichloroacetate diminished the rate of inactivation by ATP. 3. Pyruvate dehydrogenase phosphatase from human heart requires Mg2+ for activity and is sensitive to Ca2+ at concentrations of a few μmol/l. Similar ionic requirements of the skeletal muscle phosphatase have been demonstrated in a crude tissue extract. 4. The activity of pyruvate dehydrogenase in human adipose tissue was less than 10% of typical values in rats. This could be due to the high level of dietary fat consumed by humans, which is known to repress the enzyme activity in rats.