Acute toxicity of ammonium metavanadate solutions in normal saline (pH 6.7) or 0.1 M Tris-HCl-NaCl buffers (pH 7.2 or pH 7.8) was studied in BALB/c mice at 20 mg V/kg. Animals receiving these solutions subcutaneously started to show severe clinical signs 10-15 min postinjection and high mortality rates (45-73%) during the first 3 d. Animals dying because of vanadium toxicity did so only within the first 3 d after injection. NH4VO3-treated animals showed a tendency to increase their liver and spleen weights as compared to those receiving control solutions. Severe necrosis in lymphoid tissues (thymus, spleen, lymph nodes, and Peyer's patch), pulmonary hemorrhage, and renal acute tubular necrosis were commonly demonstrated in vanadium-treated animals. Toxicity of NH4VO3 solution in 0.1 M Tris-HCl-NaCl buffer (pH 7.8) was greatly reduced upon acidification with HCl to pH 6.1 or following boiling for 15 min (final pH of 7.7). Acidification of the solution reduced the mortality rate to 20 from 68%; however, the clinical signs were still severe. Boiling of the solution reduced the mortality rate to zero and moderated the severity of the clinical signs.