The sensitivity of facilitated-diffusion and Na⁺-dependent nucleoside transporters to inhibition by a series of novel compounds related to lidoflazine and mioflazine was investigated. Uridine transport by rabbit erythrocytes, which proceeds solely by the nitrobenzylthioinosine (NBMPR)-sensitive facilitated-diffusion system, was inhibited with apparent K_i values of less than 10 nM by lidoflazine, mioflazine, soluflazine and R73-335. These compounds also blocked site-specific [³H]NBMPR binding to rabbit erythrocyte membranes in a competitive fashion. The NBMPR-sensitive system in rat erythrocytes was also inhibited by lidoflazine, mioflazine, soluflazine and R73-335 but was two to three orders of magnitude less sensitive to inhibition than the system in rabbit erythrocytes (apparent K_i 7.3, 2.4, 5.7 and 0.1 μM, respectively). Lidoflazine, mioflazine and R73-335 exhibited a similar potency for the NBMPR-sensitive and -insensitive nucleoside transporters in rat erythrocytes. In contrast, soluflazine was 20- to 100-fold more potent as an inhibitor of the NBMPR-insensitive nucleoside transport component in rat erythrocytes (IC₅₀ of 0.08–0.2 μM) compared to the NBMPR-sensitive nucleoside carrier in these cells (IC₅₀≈10 μM). None of the test compounds were potent inhibitors of Na⁺-dependent uridine transport in bovine renal brush-border membrane vesicles. These results indicate that lidoflazine, mioflazine, soluflazine and R73-335 are selective inhibitors of nucleoside transport in animal cells and that the potency of these compounds as nucleoside transport inhibitors is species dependent.