NIH-07 open formula nonpurified diet has been the selected diet for the NTP rodent studies since 1980. This diet was formulated more than 20 years ago for reproduction, lactation and growth of rodents in breeding colonies. This diet which contains approximately 24% protein, 5% fat and 3.5% fiber may not be an optimal diet for rats in long-term studies, and some components such as protein, calcium and phosphorus ratio may be increasing the severity of chronic diseases. After 6 years of experimentation with different formulations of experimental diets, a new diet designated as NTP-2000 open formula nonpurified diet was formulated for rodents in toxicology studies. This diet is substantially different from NIH-07 diet and contains approximately 15% protein, 8.5% fat and 9.5% fiber. The purpose of this study was to compare 1) growth patterns, 2) hematology and clinical chemistry parameters, 3) organ weights and 4) incidence and severity of lesions in Fischer 344 (F344) rats fed the NIH-07 and NTP-2000 diets for 13 weeks. Groups of 25 male and female F344 rats of 6 weeks of age were fed NIH-07 or NTP-2000 diet for 13 weeks. Feed consumptions were measured at weekly intervals, water consumptions were determined at 4, 8 and 12 weeks, hematology and clinical chemistry evaluations were done at 12 weeks. After 13 weeks of treatment the rats were necropsied, major organs were weighed and histopathologic changes in major organs were evaluated by light microscopy. The growth patterns were essentially same for males and females fed both diets. The feed consumptions were slightly higher and water consumptions were slightly lower for rats fed NTP-2000 diet. The BUN values for males and females fed NTP-2000 diet were significantly lower due to lower protein content of this diet. The cholesterol level in males, and 5'nucleotidase and alkaline phosphatase levels in males and females fed NTP-2000 were slightly higher due to higher fat content of the diet. However, the triglyceride levels in males and females fed NTP-2000 diet were lower than the rats fed the NIH-07 diet even though the fat content of NTP-2000 diet was higher than in the NIH-07 diet and this difference may be due to higher fiber content of the NTP-2000 diet. The serum zinc and phosphorus levels of the male rats fed NTP-2000 diet were lower possibly due to lower protein, cystine and phosphorus concentrations of the NTP-2000 diet. The liver and kidney weights of male and female rats fed NTP-2000 diet were significantly lower than the rats fed NIH-07 diet and the difference is due to lower protein content of the NTP-2000 diet. The adrenal weights of rats fed NTP-2000 diet were significantly lower than the rats fed NIH-07 diet and the reason for this difference is not known. However, two-year studies with experimental diets similar to NTP-2000 diet caused a significant decrease in the incidence of adrenal pheochromocytomas and lower adrenal weights may be related to this effect. The most common lesions of F344 rats in 13-week studies are nephrocalcinosis (mineralization at corticomedullary junction of the kidney) in the females, initial changes of nephropathy such as renal tubule regeneration in males and cardiomyopathy in both sexes. The NTP-2000 diet prevented nephrocalcinosis, decreased the incidence and severity of nephropathy and decreased the severity of cardiomyopathy. In conclusion, the NTP-2000 diet 1) is adequate for growth and maintenance of rats, and 2) appears to decrease the incidence, severity or delay the development of diet and age associated lesions in rats.