The administration of cocaine hydrochloride intraperitoneally (25 mg/kg) produces a drop in VO2 in both normoxic and hypoxic mice. The critical PO2 is also decreased and so is the body temperature. The mitochondrial respiration shows a large fall in ST3 and RCR. The addition of cocaine in-vitro to the incubating medium induces changes in the mitochondrial respiration similar to those found after in-vivo administration. This report shows that in addition to its in-vivo actions cocaine alters the respiratory function of the isolated liver mitochondria. © 1991.