The object of this study was to examine the endocrine control of the serous tubules which, in addition to mucous acini, constitute the secretory portion of the submaxillary gland of the rat. To assess the respective roles of testis and thyroid, young adult male rats, which had been thyroidectomized and castrated, were treated daily for 47 days with 0.5 mg. of testosterone or 6 [mu]g. of thyroxin or both, and the serous tubules were examined by histometric methods. Control animals, in which testes and thyroids are extirpated, showed an atrophy of the serous tubules. Treatment of such animals with testosterone alone had little or no effect on the size and granule content of the cells of the serous tubules. Treatment with thyroxin alone has little effect on the size and granule content of the cells of the serous tubules, but it restored the number of these cells to normal. A combined treatment with both hormones restored a normal size and granule content as well as a normal number of the cells of the serous tubules. The testis and thyroid hormones in the doses indicated exerted a synergistic action on kidney and heart as well as on the serous tubules of the submaxillary gland, but not on the male secondary sex organs, such as seminal vesicles, nor on the metabolic responses to thyroxin, such as the increase in O2 consumption and body wt. In comparison with the major role of the testis and thyroid hormones in the maintenance of the serous tubules, nervous influences[long dash]such as those occurring in the course of feeding and fasting[long dash]have only a minor effect.