Implantation of a mammotropic tumor (MtTF4), secreting growth hormone, prolactin, and corticotropin, in female rats of Fischer F344 strain causes hypertension, vasculitis, renal and cardiac hypertrophy, and extensive renal and cardiac lesions. When rats of the same strain were implanted with the MtTF4 tumor but sodium was withheld from the diet, systolic blood pressure rose more slowly but by six weeks reached the same values recorded in the animals implanted with the tumor and allowed to consume sodium ad libitum. In the rats, on sodium deficient diet, however, the vascular damage as well as the renal and cardiac lesions were minimal or absent. Implantation of the tumor caused adrenal cortical dysfunction, and elevated levels of deoxycorticosterone were seen in the peripheral plasma of the rats of all three groups. Nonetheless, plasma deoxycorticosterone was significantly lower in rats on a sodium deficient diet as compared with those having sodium added to the diet. Light microscopic and ultrastructural studies of the adrenal glands revealed that the lack of dietary sodium largely prevented the extensive damage of the zona fasciculata cells usually seen in the tumor-bearing rats, consuming sodium ad libitum. Both hypertensive MtT tumor-bearing animals and normotensive controls on a sodium deficient diet had a conspicuous increase of renal content of renin. It is evident that hypertension may be produced in rats bearing the MtTF4 tumor even in the virtual absence of dietary sodium. It does not appear that the hypersecretion of renal renin sustains the hypertension in these rats, since high levels of this substance were seen in the kidney of normotensive controls on the same sodium deficient diet. Elevated levels of plasma DOC may possibly explain the hypertension. In addition, it is likely that the animals may also have elevated levels of glucocorticoids.