The activity of pyruvate dehydrogenase was assayed in extracts of rat hearts perfused in vitro with media containing glucose and insulin+/-acetate+/-dichloroacetate. Dichloroacetate (100mum, 1mm or 10mm) increased the activity of pyruvate dehydrogenase in perfusions with glucose or glucose+acetate. Evidence is given that dichloroacetate may facilitate the conversion of pyruvate dehydrogenase from an inactive (phosphorylated) form into an active (dephosphorylated) form.