The renal excretion of 3'-azido-3'-deoxythymidine (AZT) was investigated in clearance experiments in anesthetized rats. AZT was filtered freely and subsequently secreted, reabsorption being of minor importance. Probenecid, the classical inhibitor organic anion secretion, decreased AZT secretion. AZT, on the other hand, decreased p-aminohippurate secretion. These observations demonstrate that AZT is secreted by the organic anion secretory mechanisms. Cimetidine, an organic cation, also decreased AZT secretion at a concentration which did not inhibit the secretion of the organic anion, p-aminohippurate. This suggests that AZT might also be transported by the mechanisms secreting organic cations. However, as AZT did not inhibit the secretion of the organic cation tetraethylammonium, it cannot be concluded that AZT is transported by the mechanisms secreting organic cations.