Mitochondria were isolated from the livers of male weanling and adult Wistar strain albino rats fed low-protein diet ad libitum for 30 and 84 days, respectively. Control animals consisted of adult and weanling rats fed purina chow for the same periods. Spectrophotometric estimations of mitochondrial passive transport of protons by following changes in matrix volume at 520 nm revealed that the mitochondria of protein-deficient rats are more permeable to protons than those of control rats; the mitochondria of protein-deficient weanling rats being more permeable than mitochondria of protein of malnourished adult rats. Similar results were obtained when a sensitive pH-glass electrode was used to monitor mitochondrial proton translocation. Although protons were slowly ejected from the mitochondrial matrix of protein-deficient rats, there was a significantly high rate of influx of the ion into the matrix of these mitochondria when compared with controls. Furthermore, FCCP, a classical protonophore carried protons less rapidly across the mitochondrial membrane of malnourished animals thus indicating that a small pH difference probably exists across the mitochondrial energy-coupling membrane of protein-deficient rats. Consequently, the rates of mitochondrial Ca(2+)-translocation were lower in protein-deficient animals than in normal rats. These defects are probably due to diet-induced looseness of mitochondrial bi-layer structure.