1. The mitochondrial content of active (dephospho) pyruvate dehydrogenase (PDHA) was found to be severalfold higher at an extramitochondrial Ca2+ concentration of 2 microM (pCa6) than at pCa7. The nature of the respiratory substrate did not affect this finding. 2. This Ca2+-dependence was shown in state-4 and 50%-state-3 conditions [see Chance & Williams (1956) Adv. Enzymol. 17, 65-134], but was absent in the presence of excess ADP (state 3). 3. Na+ and Mg2+ ions shifted the pCa value required for a maximal PDHA content to lower values. This was attributed to a stimulation of mitochondrial Ca2+ egress and an inhibition of uptake, respectively. Na+ ions diminished pyruvate dehydrogenase phosphate phosphatase activity in mitochondria which had been extensively depleted of Ca2+ ions by incubation with EGTA, raising the possibility of a direct inhibitory effect of Na+ ions, unrelated to Ca2+ movements. 4. Mg2+ ions lowered the mitochondrial PDHA content at pCa 6.24 and 6.48, but had only minimal effects in the presence of EGTA. 5. The effects of P1 and bicarbonate ions on PDHA content were also studied, as possible effectors of mitochondrial Ca2+ transport. Bicarbonate ions abolished the response to Ca2+ ions, by generating maximal values of PDHA content, but such a response was still observed when physiological concentrations of both P1 and bicarbonate were used. 6. The pCa of the medium in the range 6.33 to over 7 affected PDHA content, with only very minor changes in state-4 rates of O2 uptake and no change in [ATP]/[ADP] ratio or in mitochondrial [NADH]/[NAD+] ratio, provided that Mg2+ ions were present. Thus the effect of Ca2+ ions on PDHA content is unlikely to be mediated by changes in [ATP]/[ADP] and [NADH]/[NAD+] ratio and is more likely to be direct. Equally, changes in the [acetyl-CoA]/[CoA] ratio in response to Ca2+ ions when the substrate was pyruvate were the converse of those required to mediate changes in interconversion, and are probably secondary to changes in PDHA content.