The authors report the results from Murphy and Pattee's method 9, 10 1964, 1966 for T4 and those from Hamolsky et al.'s method 5 1957 for percent retention of T3 as applied to the normal rat serum. For that purpose, 32 adult Wistar rats, of both sexes, kept at the laboratory environment and fed with usual diet were used. Blood samples were taken after noon directly from the heart, without anesthesia. The analysis showed a T4 value of 4.20 +/- 0.92 mug/100 ml of serum (both sexes). The percentuals for mean T3 retention values were found to be 87.46 +/- 6.59% (male) and 81.78 +/- 5.22% (female). No statistically significant correlation could be drawn between the body weights and the hormonal findings.