The proximal segment of the winter flounder, Pseudopleuronectes americanus, was investigated. Isolated tubules net secrete fluid, although at low rates, 37 pl X min-1 X mm-1. The dominant ions in secreted fluid are Na and Cl, with [Cl] significantly higher than in the bath. Mg and SO4 concentrations in secreted fluid are more than 10-fold greater than in the bath. The transepithelial voltage (-1.9 mV) and resistance (26 omega X cm2) indicate an electrically leaky epithelium, and transepithelial diffusion potentials suggest the Na selectivity of the paracellular pathway. Transepithelial electrochemical potentials point to active transport of Mg, SO4, and probably also Cl and to transepithelial equilibrium of Na. Failure to observe any secretory transport in tubules perfused in vitro at the commonly used perfusion rates illustrates the necessity of low, preferably subnanoliter, perfusion rates in detecting and studying low-capacity epithelial transport systems by the method of Burg.