We followed the effects of sera from unilaterally nephrectomized (uninephectomized) rats compared to sham-operated rats on <sup>3</sup>Η- p-aminohippurate (<sup>3</sup>H-PAH) and <sup>14</sup>C-tetraethylammonium (<sup>14</sup>C-TEA) uptake and oxygen comsumption (QO<sub>2</sub>) in incubating rat kidney slices. These studies were based on the assumption that a circulating renotropic substance might also influence various transport mechanisms. Sera were obtained at various times postoperation; the height of renotropic activity occurs 17–24 h after kidney extirpation. Sera removed 17–24 h postuninephrectomy significantly decreased both <sup>3</sup>H-PAH and <sup>14</sup>C-TEA uptake in incubating kidney slices. Similar to the inability to show significant renotropic activity after 36 h, sera obtained 48,96, and 168 h postuninephrectomy had no significant influence on <sup>3</sup>H-PAH and <sup>14</sup>C-TEA uptake. Addition of sera (10% v/v) generally depressed QO<sub>2</sub>. However, sera obtained from uninephrectomized rats compared to sera from sham-operated rats had relatively more depressive effects on QO<sub>2</sub> in renal tissue after 30 min of incubation (p < 0.01). No significant differences in QO<sub>2</sub> were seen when the uninephrectomized and sham-operated sera were added to the tissue immediately or after 60 min of incubation in the sera. These serum studies on <sup>3</sup>H-PAH and <sup>14</sup>C-TEA uptake simulate in many respects ones performed previously with serum from spontaneously hypertensive and salt-loaded rats and suggest the presence of a common circulating factor.