1. Phenytoin was intraperitoneally injected in six different experimental groups of adult Mongolian gerbils of both sexes at doses of 5, 100 and 150 mg/kg of body wt during 7 and 14 days. 2. Their brain, cerebellum, liver and kidney DNA, RNA and protein content were measured by conventional procedures. 3. The reference values of these rodents' cellular DNA were calculated through hepatocytes isolated following an intracardiac perfusion with collagenase in Hanks solution. 4. A concentration of 7.2 micrograms of DNA per 10(6) cells was found. 5. Phenytoin did not affect tissues of groups of animals receiving daily doses of 5 mg/kg of weight during 7 and 14 days of treatment; although a slight increase of protein concentration was observed in liver (P less than 0.01) and kidney (P less than 0.05). 6. RNA values were found to be significantly decreased in cerebellum (P less than 0.001), brain (P less than 0.05) and kidney (P less than 0.05) of those animals receiving the drug throughout 14 days. 7. DNA concentration was found to be decreased in all four tissues obtained from animals treated with daily doses of phenytoin of 100 (P less than 0.001) and 150 (P less than 0.001) mg/kg of body wt throughout 14 days. 8. These results indicate that phenytoin affected tissue cellularity only when administered at high doses and 14 days of interval of drug administration (P less than 0.001).