Exposure to mercury vapors for an hour per working day over a period of 13 years produced in a thermometer manufacturer severe signs and symptoms of mercury poisoning. Complete disability developed insidiously over the last six months of employment. During the first two months of observation, the patient was treated in succession with three chelating agents: 2,3-dimercapto-l-propanol (BAL), D-penicillamine and sodium diethyldithiocarbamate (Dithiocarb). Each agent was administered initially for a period of approximately two weeks. A second course of therapy with BAL was administered for three days. Of the three complexing agents used, BAL gave the most dramatically favorable clinical response and yielded the highest urinary excretions of mercury. Dithiocarb was partially effective; d-pencillamine proved to be essentially ineffective. Analyses of the patient's sweat indicated that appreciable amounts of mercury were excreted by this route. Following the alleviation of the severe symptoms by BAL, the patient was placed on a regimen of daily sweats and physio-therapy for a protracted period of several months. On this latter regimen, the mercury levels in the urine, blood serum and sweat were decreased to within the normal ranges of values. The patient made a complete and uneventful recovery. In patients encountering psychotic and neurological disorders of undetermined etiology, consideration should be given to unsuspected or masked chronic exposure to mercury vapors as a possible cause.