Esterase, "aliesterase" was determined, in blood sera from 38 rats, 6 hamsters, 34 guinea-pig and 8 rabbits by the Huggins & Lapides' method. Normal values for male and female animals were related separately. In female rat, blood aliesterase is higher within a mean value of 11.6 U/ml ± 4.01 and standard error of 0.83. Aliesterase sera values in male are lower than in female. The mean value being 6.9 U/ml ± 0.99, standard error, 0.26. The significance of means (t) for female and male rats was 4.4. This fact reveals that aliesterase concentration in sera is related with estrogen production or metabolism. For guinea-pig the mean values were 4.50 U/ml ± 0.15, standard error 0.036 (female) and 4.28 U/ml ± 0.30, standard error. 0.073 (male), the significance value (t) being 2.6. The rabbits showed a mean value of 4.41 U/ml ± 0.16, standard error, 0.058, while the hamsters presented 4.10 U/ml ± 0.085, standard error. 0.049 for the males and 3.81 U/ml + 0.227, standard error. 0.133 for females. Castrated animals showed a progressive decrease in the serum aliesterase. Results obtained with castrated and estrogens treated animals will be related in a further paper.