Membrane potential of mouse lymphocytes was investigated in the presence and absence of cyclosporin A (CsA) and cyclosporin G (CsG) by flow cytometry and fluorescence spectroscopy. A carbocyanine dye, dihexyloxacarbocyanine iodide [DIOC6(3)], was applied as a membrane potential probe. A dose-dependent decrease in the membrane potential of T and B lymphocytes was observed in the presence of CsA and CsG. However, pretreatment of lymphocytes with insulin reduced the effect of the cyclosporins. Mobile ionophores, such as valinomycin, ionomycin and A23187 were less effective in changing the membrane potential of lymphocytes in the presence of CsA. The channel forming ionophore, gramicidin or high extra-cellular potassium concentration (160 mM) strongly reduced the membrane potential regardless of the absence or presence the CsA. These observations suggest incorporation of CsA into the cytoplasmic membrane causing changes in ion fluxes. Other reported biochemical effects of CsA may be secondary to the observed membrane potential changes. The membrane potential change induced by CsA may have selective biological consequences in a certain subpopulation of lymphocytes.