FOM-Na solution in distilled water for injection, J.P., with its pH adjusted to 7.0 +/- 0.2 with diluted hydrochloric acid, was administered to rats and rabbits for subacute toxicity test. The results revealed the following: 1. It was intraperitoneally administered to Wistar rats each weighing 100 +/- 10 g at their age of 5 weeks, and intravenously into auricular veins of male albino rabbits each weighing about 3 kg, for 35 successive days except Sundays through one administration per day. There was no death in rats of both sexes with the doses less than 1,000 mg/kg, while 3/10 males and 5/10 females died with the dose of 2,000 mg/kg. In terms of general conditions, both stretched physical position and vocalization were noted, which were presumed to be attributable to the stimuli of the administration. In the postmortem examination mutual adhesion of organs in the peritoneum was noted, while in the lightmicroscopic examinations histological proliferation and adhesion were found out in the hepatic capsules or serous membrane of the intestine etc., but no abnormalities were detected in the other organs. In the mean body weights, there were no significant differences between the control group and the groups of males with doses less than 500 mg/kg and females with doses less than 1,000 mg/kg whereas in the groups of males with doses more than 1,000 mg/kg and the females with a dose of 2,000 mg/kg, a trend of reduced weight gain was noted in comparison with the control group. The feed intake also was reduced as the dose was elevated. In terms of the male group hematology, the In. P increased with doses higher than 250 mg/kg, while BUN was reduced in the groups with a dose of 250 mg/kg and doses higher than 1,000 mg/kg, and Na was reduced in the groups with doses from 125 up to 500 mg/kg. In the female groups, the loss of Hgb and rise in the Cl were noted in the doses higher than 500 mg/kg while the loss of WBC was noted in almost all the treated groups. However, none of these changes was suggestive of specific abnormalities when compared with the photomicroscopic findings and our hematological background data. 2. There were no significant changes in the general conditions of any group of rabbits. Their mean body weights and their mean feed intakes proceeded almost similarly with those of the control group. In the hematological and histopathological tests also, no specific abnormal finding was experienced, which were deemed to be attributable to the administration of FOM-Na.