Tetraethylammonium (TEA) transport was studied in isolated perfused snake (Thamnophis spp.) proximal renal tubules. Unidirectional lumen-to-bath (J1----bTEA) and bath-to-lumen (J1----bTEA) fluxes exhibited saturation kinetics, but Jb----1TEA also exhibited an apparent diffusive component and J1----bTEA did not. Jb----1TEA exceeded J1----bTEA at all concentrations studied, resulting in net TEA secretion. Transport into cells across both luminal and peritubular membranes was apparently against an electro-chemical gradient and was inhibited by cyanide. Km for J1----bTEA (5.9 microM) was about one-third Km for Jb----1TEA (19.9 microM), indicating greater affinity of the luminal transporter for TEA; but Vmax for Jb----1TEA (153 fmol X min-1 X mm-1) was about six times Vmax for J1----bTEA (27 fmol X min-1 X mm-1), indicating a greater capacity of the peritubular transporter for TEA, which could account for net TEA secretion. Jb----1TEA was inhibited by N-methylnicotinamide (NMN) in the bath, but J1----bTEA was inhibited initially and then apparently transstimulated by NMN in the lumen, indicating possible countertransport. J1----bTEA, but not Jb----1TEA, was significantly reduced by replacement of sodium with sucrose, indicating possible sodium dependency of the luminal transporter. All data indicate active (either primary or secondary) TEA transport at both luminal and peritubular membranes but net transepithelial transport in the bath-to-lumen direction.