The mitochondria isolated from the muscles of fish acclimated to grow in different salinities have been studied with reference to their Ca2+ uptake capacity and compared to those isolated from fresh-water fish muscle. The results show a drastic response by the mitochondria with reference to their Ca2+ uptake function soon after exposure to the stress. Evidence is also presented to suggest an alteration in conformation. This perturbation appears to be the initial response to the stress since the normal state (as that of the fresh-water fish) is restored in course of time. Further, so far there is no indication that the electron transport function and ATP production are affected by the ionic stress conditions. This would support the physiological relevance of the mitochondrial capacity for Ca2+ uptake.