Report of a case of acute intoxication by inorganic arsenic with a picture of peripheral polyneuropathy. High concentrations of arsenic were found in the peripheral nerve studied (1.48 mg/100 g fresh tissue; 0,40 mg/100 g dry tissue) blood and urine. Before the treatment with penicillamine the urinary concentration of arsenic was 80,0 micrograms/100 ml; 7 days after the onset of treatment the excretion increased to 150.2 micrograms/100 ml. Improvement of the clinical conditions was parallel to these biochemical results.