The kinetic characteristics of the transport of Ca2+ in the human term placental mitochondria previously depleted of their intramitochondrial calcium, was studied. Maximal Ca2+ uptake was observed in the presence of inorganic phosphate and ADP. Ca2+ transport showed a Michaelis-Menten behavior, with a Vmax of 61 nmol mg-1 min-1 and a Km of 118 microM. Ruthenium red inhibited mitochondrial Ca2+ uptake. Under optimal conditions of Ca2+ uptake, the cation inhibited progesterone synthesis in placental mitochondria. The kinetic parameters of Ca2+ transport agree with the physiological role of mitochondria in the human placenta.