Transients in the potential difference spontaneously developed by theNecturus proximal tubule were characterized during and after voltage or current clamp commands. These voltage transients were adequately fitted by an exponential function similar to that describing the ionic charging of a leaky fluid capacitance and were slower during clamp periods (t 1/2=0.98 min) than after release of the clamp (t 1/2-0.46 min). Changes in luminal ionic composition and cellular membrane potential were ruled out as sources of generation of the voltage transients. The volume of the fluid compartment in which concentration changes occurred was calculated from the electrical data and it was concluded that the extracellular shunt path was the principal site of the concentration changes which resulted in voltage transients. A fall in transepithelial resistance to nearly one-half its original value occurred during hyperpolarizing commands while depolarizing commands did not significantly alter resistance. The resistance changes were interpreted as indicative of the degree of widening of the lateral intercellular spaces caused by fluid accumulation or depletion. The important role of the lateral space dimensions in determining epithelial permeability, electrical resistance and voltage transients was pointed out, and a new electrical analogue model of the shunt path was proposed.