Thirteen esterase isozymes were separated from kidney extracts of normal adult Syrian hamsters ( Mesocricetus auratus) using the zymogram technique with vertical starch gel electrophoresis and naphthyl esters as substrates. The activities of two esterases were altered by gonadectomy and by treatment with testosterone or estradiol-17β but not with progesterone. One of these esterases showed negligible activity in castrated males and in normal and spayed females. The activity of this esterase increased appreciably in castrated males treated with testosterone or estradiol and in testosterone-treated spayed females. This enhanced activity, however, was not consistent in spayed females treated with estradiol. No sex differences in renal esterases were observed in immature hamsters but a variation was detected in normal adults. The difference appears to be of normal enzyme levels rather than the absence of any esterase. Under the influence of estradiol and progesterone, separately or in combination, distinct alterations in kidney esterase profiles were observed. Certain other quantitative hormonal effects were found. These hormone-dependent esterases do not appear to be cholinesterases, as they were unaffected by eserine. The marked sensitivity of various hamster kidney esterases to estrogen appears to be a unique response and may be related to the estrogenic induction of renal adenocarcinoma, initiated under similar conditions, but not found with androgen-treated or other estrogenized animals. These findings indicate that gonadal effects on the hamster kidney may be of a more complex nature than has been considered heretofore.