Either L-methionine (L-met) or L-cysteine (L-cys), given alone and in combination with Prussian blue (PB) was characterized as treatment against acute thallium (Tl) toxicity in rats. Animals were intoxicated with 32 mg/kg Tl acetate corresponding to rat LD50. Antidotal treatments were administered during 4 days, as follows: (1) vehicle, (2) L-met 100 mg/kg i.p. twice a day, (3) L-cys 100 mg/kg i.p. twice a day, (4) PB 50 mg/kg oral, twice a day, (5) L-met + PB and (6) L-cys + PB. Mortality was as follows: control 50%; L-met 80%; L-cys 80%; PB 20%; L-met + PB 90% and L-cys + PB 100%. In a different experiment, using 16 mg/kg of Tl, tissue levels of this metal were analyzed. PB treatment statistically diminished Tl content in body organs and brain regions (P < 0.01). Whereas, separate treatments of L-met and L-cys failed to decrease Tl content in organs and brain regions; while its administration in combination with PB (L-met + PB and L-cys + PB groups) lowered Tl levels in body organs in the same extent as PB group. Results indicate that L-met and L-cys administered alone or in combination with PB should not be considered suitable treatments against acute Tl toxic effects because this strategy failed to prevent mortality and Tl accumulation in brain.