Tri- and pentavalent antimony were quantified in Leishmania mexicana pifanoi amastigotes and promastigotes by atomic absorption spectroscopy with electrothermal atomization. Leishmania grown in axenic culture were treated with either potassium antimony tartrate [Sb(III)] or sodium stibogluconate ([\mathbf{S b}(\mathrm{V})]). The parasites were collected, digested with nitric acid, and subjected to atomic absorption spectroscopy. The method was linear from 0 to (7 \mathrm{ng}) of antimony. The interassay coefficients of variation were 9.6 and (5.7 %(N) (=5) ) for 0.52 and 3.7 -ng samples of leishmanial antimony, respectively. The limit of detection was (95 \mathrm{pg}) of antimony. The assay was used to characterize (\mathrm{Sb}) (III) and (\mathrm{Sb}(\mathrm{V})) influx and efflux kinetics. Influx rates were determined at antimony concentrations that produced a (50 %) inhibition of growth ( (\mathrm{IC}_{\mathrm{5o}}) ). The influx rates of (\mathrm{Sb}(\mathrm{V})) into amastigotes and promastigotes were 4.8 and (12 \mathrm{pg} / \mathrm{million}) cells (/ \mathrm{h}), respectively, at (200 \mu) g antimony/ ml. The influx rate of (\mathrm{Sb}() III) into amastigotes was 41 (\mathrm{pg} / \mathrm{million}) cells (/ \mathrm{h}) at (20 \mu \mathrm{g}) antimony/ml. Influx of Sb(III) into promastigotes at (1 \mathrm{\mu g}) antimony/ml was rapid and reached a plateau of (175 \mathrm{pg} /) million cells in 2 h. Efflux of (\mathrm{Sb}(\mathrm{III})) and (\mathrm{Sb}(\mathrm{V})) from amastigotes and promastigotes exhibited biphasic kinetics. The initial ((\alpha)) half-life of (\operatorname{Sb}(V)) efflux was less than (4 \mathrm{~min}) and that of (\mathrm{Sb}(\mathrm{III})) was 1-2 (\mathrm{h}). The apparent terminal ((\beta)) half-lives ranged from 7 to 14 h. 1993 Academic Press, Inc.