This study examined the effect of the nonsteroidal anti-inflammatory drug ibuprofen on liver inner mitochondrial membrane permeability transition in the presence of Ca2+ and phosphate. Incubation of isolated liver mitochondria with ibuprofen (0.1-0.4 mM) induced inner mitochondrial membrane permeability as indicated by loss of inner mitochondrial membrane potential, swelling of matrix and loss of pre-accumulated Ca2+. The presence of cyclosporin A (1 microM) in the incubation medium preventing ibuprofen from causing loss of inner mitochondrial membrane potential, swelling and loss of pre-accumulated Ca2+. It is concluded that ibuprofen acted as an activator of Ca2+ and phosphate in promoting the opening of inner mitochondrial membrane pore.