Recently, uptake2 was shown to exist in the clonal Caki-1 cell line. The aim of this study was two-fold: a) to determine, in Caki-1 cells, the intracellular fate of 3H-noradrenaline after its translocation by uptake2 and b) to analyse the force driving uptake2. Caki-1 cells have the characteristics of a "metabolizing system" in which the activity of catechol-O-methyl transferase (COMT) greatly exceeds that of monoamine oxidase (MAO). In all subsequent experiments these enzymes were inhibited. The determination of initial rates of uptake2 into Caki-1 cells at an extracellular pH between 6.9 and 7.9 indicated that the protonated species of 3H-noradrenaline is transported. Depolarization of Caki-1 cells (by three different procedures) inhibited the inward transport. Determination of the time course of the specific accumulation of 3H-noradrenaline in Caki-1 cells and of 3H-isoprenaline in the perfused rat heart (both mediated by uptake2) revealed that depolarization (by high K+) reduced the rate constant for inward transport (kIN) and increased that for outward movement (kOUT). Consequently, depolarization reduced the steady-state factor of accumulation. It is proposed that, as the protonated species of the substrates of uptake2 is transported, the membrane potential is likely to provide the driving force for uptake2. The fact that depolarization decreased kIN and increased kOUT agrees with this proposal, as do the magnitudes of the steady-state accumulation factors determined in Caki-1 cells and perfused rat heart.