Posttreatment effects of phenobarbital (30 mg/kg I.P. per day for 5 days) on liver mitochondrial TEM ultra-structure in adult rats were studied. Liver mitochondria population samples in treated rats exhibited pleomorphic morphological dumbbell shapes (7%), U-type (crescent) shapes (4%) and O-type (ring-like) shapes (4%). This pathology was not observed in vehicle-treated animals. Phenobarbital is a drug that induces within the rat liver increased protein synthesis in the mitochondria and mitochondrial morphological shape changes.