The regulatory properties of the Ca2+-sensitive intramitochondrial enzymes (pyruvate dehydrogenase phosphate phosphatase, NAD+-isocitrate dehydrogenase and 2-oxoglutarate dehydrogenase) in extracts of rat kidney mitochondria were found to be essentially similar to those described previously for other mammalian tissues; in particular each enzyme could be activated severalfold by Ca2+ with half-maximal effects (K0.5 values) of about 1 microM and effective ranges of approx. 0.1-10 microM Ca2+. In intact mitochondria prepared from whole rat kidneys incubated in a KCl-based medium containing respiratory substrates, the amount of active, nonphosphorylated pyruvate dehydrogenase could be increased severalfold by increases in extramitochondrial [Ca2+]; these effects could be blocked by ruthenium red. Similarly, Ca2+-dependent activations of NAD+-isocitrate dehydrogenase and 2-oxoglutarate dehydrogenase could be demonstrated in intact, fully coupled, rat kidney mitochondria by either following O2 uptake (in the presence of ADP) and NAD(P)H reduction (in the absence of ADP) on presentation of non-saturating concentrations of either threo-Ds-isocitrate or 2-oxoglutarate, respectively, under appropriate conditions, or for the latter enzyme only, also by following 14CO2 production from 2-oxo[1-14C]glutarate (in the absence or presence of ADP). Effects of Na+ (as a promoter of egress) and Mg2+ (as an inhibitor of uptake) on Ca2+-transport by rat kidney mitochondria could be readily demonstrated by assaying for the Ca2+-sensitive properties of the intramitochondrial Ca2+-sensitive dehydrogenases within intact rat kidney mitochondria. In the presence of physiological concentrations of Na+ (10 mM) and Mg2+ (2 mM), activation of the enzymes was achieved by increases in extramitochondrial [Ca2+] within the expected physiological range (0.05-5 microM) and with apparent K0.5 values in the approximate range of 300-500 nM. The implications of these results on the role of the Ca2+-transport system of kidney mitochondria are discussed.