Since the curing effect of massive doses of myoinositol on fatty liver provides the possibility of its clinical application, the effect of massive doses of myoinositol on the liver and kidney of rat was studied morpho logically. Male albino rats weighing about 100g were injected with 3.0g of myo inositol per kg body weight per day for 30 days. No appreciable change was detected in these organs when examined under a light microscope. The same result was obtained when administered per os. These con clusions were also good in the cases of female rats.In the previous publication from this laboratory (1), the beneficiary effect of myoinositol on dietary fatty liver and its mechanism have been reported. It was also shown how the administered myoinositol gets incorporated into phosphatidylinositol and brings about improvement in the liver parenchymal cells of animals suffering from fatty liver (2).It was thought fit at this time to investigate any toxic effect of massive doses of myoinositol to facilitate its clinical applications, even though myoinositol is a naturally occurring substance and is approved as a drug. It was especially desirable to investigate the toxic effects of prolonged administration of this cyclitol. Since it has been reported that dextran (3) provokes hydropic changes in the tubules of the kidney, and xylitol (4, 5) brings about hyperuricemia and hyperbilirubinemia, such effects were carefully checked with this cyclitol. Hence it was decided to examine subacute and chronic toxicity of myoinositol by means of morphological examination on the liver and kidney.