The concentration of hepatic nuclear T3 receptors was measured in isolated nuclei from athyreotic mice bearing thyrotrophic tumors and intact rats with Walker 256 carcinoma. Receptor concentration was reduced in all tumor-bearing animals. The mean receptor capacity of the Walker tumor-bearing rats [0.31 +/- 0.05 (SEM) ng/mg DNA] was significantly decreased from simultaneously assayed controls (0.47 +/- 0.04 ng/mg DNA; P less than 0.01). No change in the apparent equilibrium association constant was observed. In individual rats, the magnitude of the decrease in nuclear T3 receptor concentration was highly correlated with the decrease in tumor-free body weight. Additional studies showed that the decrease in nuclear receptors was not due to delayed equilibration of added T3 with nuclear sites in vitro or to an increase in endogenous hepatic T3 concentration. The plasma concentration of total and free T4 and T3 was decreased in tumor-bearing rats. Plasma TSH concentration, however, remained unchanged. Thus, these transplantable neoplasms seem to be associated with decreased hepatic nuclear receptors and low concentrations of plasma thyroid hormones. The unchanged plasma TSH suggests that the animals remained euthyroid.