The effects of intramitochondrial CaZ+ on the activities of the Ca*+-sensitive intramitochondrial enzymes, (i) pyruvate dehydrogenase (PDH) phosphate phosphatase, and (ii) oxoglutarate dehydrogenase (OGDH), were investigated in intact rat liver mitochondria by measuring (i) the amount of active PDH (PDH,) and (ii) the rate of decarboxylation of c(-[ 1-Yloxoglutarate (at non-saturating [oxoglutarate]), at different concentrations of extramitochondrial Ca*+. In the presence of Na+ and Mg*+, both PDH and OGDH could be activated by increases in extramitochondrial [Ca*+] within the expected physiological range (0.05 -5 PM). When liver mitochondria were prepared from rats treated with adrenaline, and then incubated in Na-free media containing EGTA, both PDH and OGDH activities were found to be enhanced. Evidence is presented that the activation of these enzymes by adrenaline is brought about by a mechanism involving increases in intramitochondrial [Ca*+].