The key role of the mitochondria in the regulation of cellular Ca2+ led to a study of mitochondrial Ca2+ uptake during the infection of chicken embryo cells with Semliki Forest virus. Mitochondrial Ca2+ uptake was stimulated during the first 5 h of infection but declined later in infection. The early stimulation suggests an increase of cytoplasmic ionized Ca2+, whereas the later decrease indicates mitochondrial injury. This functional deterioration was correlated with an increase of the permeability of the inner mitochondrial membrane. Polarographic experiments showed that electron transport is impaired, whereas transduction of energy to Ca2+ uptake is intact.