A six-month intraperitoneal chronic toxicity study of cefpirome sulfate (CPR) in rats as well as a two-month recovery study were carried out at dose levels of 51.2, 128, 320 and 800 mg/kg/day. The results are as follows. 1. CPR caused no remarkable clinical signs in the general condition of the animals. 2. Body weight gain was depressed in males and females given 800 mg/kg/day. Food consumption, however, was not afected at any dose level. Water consumption increased in males and females at dose of greater than or equal to 320 mg/kg/day. 3. Except an increase in urine volume in males at 800 mg/kg/day, no abnormalities in urinary parameters were found. 4. Frequent decreases in erythrocyte count, hematocrit and hemoglobin concentration were seen in males and females of the 800 mg/kg/day group. At this dose, certain males and females also showed an increase in the reticulocyte count. 5. The serum-biochemical examinations showed a very slight or slight decrease in total cholesterol in males and females given 320 mg/kg/day or more, and increases in uric acid and inorganic phosphorus concentration in some males and females given 800 mg/kg/day. 6. Very slight or slight increases in spleen weight in males and females at a dose of greater than or equal to 320 mg/kg/day, in thyroid weight in females at a dose of greater than or equal to 320 mg/kg/day and in kidney weight in males given 320 mg/kg/day or more as well as in females at 800 mg/kg/day were measured. Further, a decrease in thymus weight was seen in females of the 800 mg/kg/day group. 7. At autopsy, very slight or slight reddish browning of the thyroid was seen in males given 320 mg/kg/day or more and in females given 800 mg/kg/day.(ABSTRACT TRUNCATED AT 400 WORDS)