The cell-to-medium distribution ratios at steady state of L-arginine (RArg) and of the lipid-soluble cation tetraphenylphosphonium (RTPP) were studied as a function of the membrane potential (Em) in adult human fibroblasts. The relationship between RArg and Em was qualitatively similar to that of RTPP and Em. Quantitatively, RArg and RTPP differed in that 1) RTPP was much greater than RArg when Em was near zero, indicating a significant binding component in the uptake of TPP+ but not of L-arginine, and 2) after a correction for binding when Em is near zero, RTPP was still greater than RArg so that RT/F . ln RTPP exceeded RT/F . ln RArg by 10-25 mV. The pattern of the redistribution of accumulated TPP+ and arginine after an alteration of Em was identical. In null-point experiments, the external [K+] for which there were no changes in cellular TPP+ or L-arginine in the presence of high valinomycin (the null points) were very similar for the two probes. Em calculated from the null-point measurements (-70(-)-80 mV) was also very similar to RT/F . ln RArg and thus smaller than RT/F.ln RTPP. It was concluded that 1) there was an additional TPP+ binding as cellular [TPP] rose in response to more negative membrane potentials, 2) the transport system for L-arginine in these cells (system y+) operates as a facilitated diffusion system driven by the membrane potential, and 3) in some circumstances, L-arginine could be employed as a probe of Em.