Decoding of fast cytosolic Ca(2+) concentration ([Ca(2+)](i)) transients by mitochondria was studied in permeabilized cat ventricular myocytes. Mitochondrial [Ca(2+)] ([Ca(2+)](m)) was measured with fluo-3 trapped inside mitochondria after removal of cytosolic indicator by plasma membrane permeabilization with digitonin. Elevation of extramitochondrial [Ca(2+)] ([Ca(2+)](em)) to >0.5 microM resulted in a [Ca(2+)](em)-dependent increase in the rate of mitochondrial Ca(2+) accumulation ([Ca(2+)](em) resulting in half-maximal rate of Ca(2+) accumulation = 4.4 microM) via Ca(2+) uniporter. Ca(2+) uptake was sensitive to the Ca(2+) uniporter blocker ruthenium red and the protonophore carbonyl cyanide p-trifluoromethoxyphenylhydrazone and depended on inorganic phosphate concentration. The rates of [Ca(2+)](m) increase and recovery were dependent on the extramitochondrial [Na(+)] ([Na(+)](em)) due to Ca(2+) extrusion via mitochondrial Na(+)/Ca(2+) exchanger. The maximal rate of Ca(2+) extrusion was observed with [Na(+)](em) in the range of 20-40 mM. Rapid switching (0.25-1 Hz) of [Ca(2+)](em) between 0 and 100 microM simulated rapid beat-to-beat changes in [Ca(2+)](i) (with [Ca(2+)](i) transient duration of 100-500 ms). No [Ca(2+)](m) oscillations were observed, either under conditions of maximal rate of Ca(2+) uptake (100 microM [Ca(2+)](em), 0 [Na(+)](em)) or with maximal rate of Ca(2+) removal (0 [Ca(2+)](em), 40 mM [Na(+)](em)). The slow frequency-dependent increase of [Ca(2+)](m) argues against a rapid transmission of Ca(2+) signals between cytosol and mitochondria on a beat-to-beat basis in the heart. [Ca(2+)](m) changes elicited by continuous or pulsatile exposure to elevated [Ca(2+)](em) showed no difference in mitochondrial Ca(2+) uptake. Thus in cardiac myocytes fast [Ca(2+)](i) transients are integrated by mitochondrial Ca(2+) transport systems, resulting in a frequency-dependent net mitochondrial Ca(2+) accumulation.