The experiment was carried out to investigate the effects of total parenteral nutrition (TPN) on body weight, nitrogen balance and the histological or cytological changes of main organs in rats. TPN was introduced to 59 female Wistar rats of seven weeks after birth, for ten days. The animals were devided into six groups as follows: Group I Control, fed orally by pellets Group II TPN of amino acid-fat-glucose Group III TPN of amino acid-glucose Group IV Fat and glucose Group V Amino acid exclusively Group VI Starvation 1) The body weight increases in Group II and III were 6.7% and 3.6% of the initial value, respectively. However, the increase in body weight of each group was less than that of control. Nitrogen balance study showed that cumulative nitrogens of Group II and III for seven days were 1.10 and 0.93 grams, respectively. 2) Fatty infiltration in the liver was not due to whether TPN consisted of fat or not. When a Cal/N ratio was increased in TPN, liver showed fatty infiltration at the peripheral regions of the lobules. The fatty infiltration seemed to become more diffuse as caloric excess was greater. Electron microscopy of hepatocytes revealed multiple giant lipid droplets in cytoplasm and glycogen cluster localizing around lipid droplets. This suggests that glycogen may change to fat following TPN. 3) TPN caused an atrophy of exocrine pancreas. The atrophy was much greater in Group IV, which was excluded amino acid. Ultrastructurally, pancreatic acinar cells revealed increased FCD (focal cytoplasmic degradation) of various size, augumented flat mitochondria, increased immature zymogen granules and dilatation of rough surfaced endoplasmic reticulum. These results show that deprivation of amino acid from TPN causes impairment to pancreatic acinar cells, even in short period. 4) Infusion of hypertonic glucose or amino acid caused cloudy swelling and vacuolization of renal tubule epithelial cells, which ultrastructurally coincided with the swelling, deformity and vacuolization of mitochondria, low electron density of cytoplasm and increased lysosomes.