These studies indicate that tetraethylammonium (TEA) is transported by saturable processes from bath to lumen and lumen to bath in isolated, perfused snake (Thamnophis spp.) proximal renal tubules and that the unidirectional flux from bath to lumen (Jᵇ¹(TEA)) exceeds the unidirectional flux from lumen to bath (J¹ᵇ (TEA)) at all TEA concentrations studied. In order to examine the transport process further, the effects of N¹-methylnicotinamide (NMN), temperature, sodium cyanide, and the removal of Na⁺ on TEA transport were studied. Steady-state Jᵇ¹(TEA) (103.2 ± 1.1 fmoles min⁻¹ mm⁻¹ at {TEA}(b) = 8.1 μM) was inhibited by 50% at {NMN}(b) = 4.0 mM. When TEA and NMN were present in the lumen, J¹ᵇ (TEA) was depressed initially (T < 8 min). However, at steady-state, the presence of NMN in the lumen appeared to stimulate J¹ᵇ (TEA). This suggests a trans-stimulation effect of NMN on J¹ᵇ (TEA). During some flux experiments tubules were perfused at room temperature (24°C) and then cooled to 2.8°C. Jᵇ¹(TEA) decreased approximately 25% when compared to control values. This effect was reversible. Other tubules were perfused in the presence of 2.5 mM sodium cyanide in the perfusate and bath. Cellular concentrations of TEA dropped from 67 to 27 times the bath concentration during Jᵇ¹(TEA) measurements and from 21 to 6 times the mean luminal concentration during J¹ᵇ (TEA) measurements. This indicates that little if any intracellular binding of TEA occurs, as passive accumulation should be 16.1 times the bath or lumen TEA concentration (assuming a -70 mV PD). To determine if TEA transport was dependent on the presence of Na⁺, Na⁺ in the bath and perfusate solutions was replaced isosmotically by sucrose. Jᵇ¹(TEA) was not significantly changed in the absence of Na⁺. J¹ᵇ (TEA) decreased to 56% of control in the absence of Na⁺. This effect was reversible. Thus, J¹ᵇ (TEA) is Na⁺-dependent and Jᵇ¹(TEA) is not. These data suggest that the transport characteristics of the carrier for TEA at the luminal and peritubular membranes of the distal-proximal tubules of garter snakes are dissimilar.