Tetracycline-induced fatty metamorphosis of the liver1) of human and experimental animals has already been reported. Einheber et al.2) demonstrated that there is increased accumulation of glyceride in the liver of mice after administration of chlortetracycline. In tetracycline-treated rat the secretion of triglyceride by the liver is impaired3) and the stimulation of triglyceride synthesis observed in liver of drug-treated animals is due to preferential utilization of free fatty acids. In the liver, triglyceride synthesis is believed to occur in microsomal tissue and soluble cytoplasm4). The fatty acid portion of the molecule is derived either from plasma or endogenously formed free fatty acids within the liver. We have demonstrated5) that in experimental animal, tetracycline enhances the release of free fatty acids from adipose tissue, resulting in, elevation of plasma free fatty acid concentration. The present study describes the effect of antibiotic on fatty acid and cholesterol biogenesis in the liver slices of tetracycline-treated rats when the drug was administered at a therapeutic level. © 1971, JAPAN ANTIBIOTICS RESEARCH ASSOCIATION. All rights reserved.