The following conclusion has been drawn from our study on the toxicological effects of FOM-Na through chronic toxicity tests with Wistar rats and Beagle dogs.<BR>I. Wistar Rats<BR>The results of continued intraperitoneal administration of FOM-Na to rats once a day for 182 successive days revealed that there was no death in both sexes of the group less than 500 mg/kg and that 3 males and 5 females of the group with 1,000mg/kg and all the animals of both sexes of the group with 2,000mg/kg died. The general conditions of these rats of both sexes were found to have been similar to those of the rats in the subacute toxicity test, with no significant differences in mean body weights between the control groups and the groups of males with doses less than 500mg/kg and the group of females with doses less than 1,000mg/kg, but with trends of suppressed weight gain rates in the groups with doses more than the above-mentioned doses comparing those of the control groups. The mean feed intakes were similar to each other of the control groups and the groups of both sexes given doses less than 1,000mg/kg, but the mean feed intake of the groups with a dose of 2,000mg/kg was less than that of the control group, and all the animals of this group eventually died. Changes were noted in TP, Ly, Ne, Cl and K among other hemato-biochemical findings. But, as there was no abnormal light-microscopic finding in any specific organ, and as the hemato-biochemical variations were not found to be particularly remarkable in comparison with our background data, these changes could hardly be attributable to the administration of FOM-Na. However, despite absence of any specific abnormality in the hematological findings, degenerative figures were noted in the renal uriniferous epithelium in the light-microscopic observations particularly in the groups with higher doses of FOM-Na. Changes related to the decrease of resistibility against infections such as pneumonia, hepatic microgranulomatic foci and chronically infected spleen were noted in each groups receiving high doses of FOM-Na.<BR>II. Beagle Dogs<BR>Continued intravenous administration of FOM-Na to Beagle dogs once a day for maximum 182 days resulted occasional vomitting of yellowish watery fluid or bubble-like substances in some animals of the group with more than 250mg/kg. But these symptoms were not always specific for administration of FOM-Na and could be often noted following intravenous injections of antibiotics. It was confirmed by the supplementary test that FOM-Na in lower concentrations and at slower injection rates did not develop these symptoms. Both the mean body weight and mean feed intake in the groups with its administration were similar to those of the control group. In the biochemical tests on serum samples, UA was found to have reduced in the group with 500mg/kg. In the electron-microscopic findings of the kidney, some products degenerative changes or vacuoles were noted in the epithelial cells of the principal tubules and the degenerations or deficiency figures were found in the epithelium of collecting tubules. But as no remarkable changes were noted in the light-microscopic findings, above observations were thought to be such a grade as would not be termed as toxic effects of FOM-Na to kidneys. No abnormalities were detected in the other organs, too.