Vasopressin or alpha-adrenergic agents such as phenylephrine or adrenaline, but not glucagon, elicited an initial decrease in flux through pyruvate dehydrogenase assayed by 14CO2 production from [1-14C]pyruvate in perfused rat liver. This rapid decrease in 14CO2 production was maximal within 1-2 min of exposure, concomitant with a rise in effluent pyruvate concentration: a subsequent return towards initial values in both parameters was completed well before 5 min. This time course was superposed with Ca2+ efflux from perfused liver, maximal (at 116 nmol/min per g wet wt. of liver) at 1-2 min of exposure. The percentage of the active (dephospho) form of pyruvate dehydrogenase was not decreased at 2 min of exposure. The effect on flux through pyruvate dehydrogenase by phenylephrine was abolished by prazosine, phentolamine or phenoxybenzamine. Ionophore A23187 also caused a depression in 14CO2 production from [1-14C]pyruvate and a rise in effluent pyruvate concentration, but this effect was stable for longer times, and it was delayed when Ca2+ was omitted from the perfusion medium. Responses of phenylephrine and A23187 were not additive. The results demonstrate that under the experimental conditions employed in intact perfused liver, the mitochondrial multienzyme system of pyruvate dehydrogenase is sensitive to vasopressin, alpha-adrenergic agents and A23187. The similar time course in Ca2+ efflux may be indicative of the involvement of Ca2+ in mediating this effect.