The mechanism of membrane permeation of several 2',3'-dideoxynucleosides was investigated at 37 degrees with human erythrocytes using an "inhibitor-stop" assay. Transport (per 5 microL cells) via the nucleoside and nucleobase carriers was assessed by inhibition of influx with dilazep and adenine, respectively. Mechanisms of cellular entry were highly individualized: 2',3'-dideoxyadenosine and 3'-deoxythymidin-2'-ene via nonfacilitated diffusion, with high rates; 2',3'-dideoxyguanosine mainly via the nucleobase carrier (Km = 390 microM, Vmax = 32 pmol/sec); 2',3'-dideoxyinosine by both nucleobase (Km = 850 microM, Vmax = 2.7 pmol/sec) and nucleoside (Km = 7.4 mM, Vmax = 16 pmol/sec) carriers, with a low rate of nonfacilitated diffusion; and 2',3'-dideoxycytidine, equally by the nucleoside carrier (Km = 23 mM, Vmax = 65 pmol/sec) and by nonfacilitated diffusion, with a low rate. These results demonstrate that the nucleobase carrier plays an important role in the influx of two of these dideoxynucleotides and that nonfacilitated diffusion is not necessarily the chief mode of membrane permeation of this class of drugs.