Abstract The membrane potential of guinea pig polymorphonuclear leukocytes has been assessed with two indirect probes, tetraphenylphosphonium (TPP +) and 3,3′-dipropylthiadicarbocyanine (diS-C 3-(5)). The change in TPP + concentration in the medium was measured with a TPP +-selective electrode. By monitoring differences in accumulation of TPP + in media containing low and high potassium concentrations, a resting potential of −58.3 mV was calculated. This potential is composed of a diffusion potential due to the gradient of potassium, established by the Na +, K + pump, and an electrogenic potential. The chemotactic peptide fMet-Leu-Phe elicits a rapid efflux of accumulated TPP + (indicative of depolarization) followed by its reaccumulation (indicative of repolarization). In contrast, stimulation with concanavalin A results in a rapid and sustained depolarization without a subsequent repolarization. The results obtained with TPP + and diS-C 3-(5) were comparable. Such changes in membrane potential were observed in the absence of extracellular sodium, indicating that an inward movement of sodium is not responsible for the depolarization. Increasing potassium levels, which lead to membrane depolarization, had no effect on the oxidative metabolism in nonstimulated or in fMet-Leu-Phe-stimulated cells. Therefore, it seems unlikely that membrane depolarization per se is the immediate stimulus for the respiratory burst.