The effect of nine monoalkyl esters of meso-2,3-dimercaptosuccinic acid (DMSA) on '03Hg retention after a single i.p. dose was evaluated in 6-7 week-old female albino rats. The monoesters were the monomethyl (MMDMS), monoethyl (MEDMS), mono-n-propyl (Mn-PDMS), monoisopropyl (Mi-PDMS), mono-n-butyl (Mn-BDMS), monoisobutyl (Mi-BDMS), mono-n-amyl (Mn-ADMS), monoisoamyl (Mi-ADMS) and mono-nhexyl (Mn-HDMS). Dimercaptosuccinic acid or one of the monoesters were administered at a dose of 0.25 mmol kg-body wt. twice, i.e. 30 niin and 24 h after '03Hg administration. The whole body (WB) radioactivity was determined on the 2nd, 4th and 6th days. The radioactivity in the carcass (C) (whole body without the gastrointestinal tract), liver (L), both kidneys ( K ) and brain (B) was determined 6 days after ""Hg administration. All treated animals had a significantly lower body burden of mercury than the controls. l h e reduction of '"-'Hg retention in WB and other body compartments was higher in animals treated with monoesters than in rats treated with DMSA. The relative effectiveness of the monoesters was dependent on the nature of the alkyl groups, the efficiency being higher in higher analogues. Maximum activity was attained with the C, (Mn-ADMS, Mi-ADMS) and Cn (Mn-HDMS) esters. These chelators reduced WB. C, L, K and B mercury retention by 90. 89, 76, 93 and 80%, respectively. [so derivatives were more efficient thail the normal isomers (Mi-PDMS > Mn-PDMS; Mi-BDMS > Mn-BDMS; Mi-ADMS > Mn-ADMS). The effect of all monoesters was higher in reducing kidney than liver mercury retention.