Studied 43 patients with methylmercury poisoning. Results indicate that 74.4% of the patients showed some degree of depression; their blood levels of mercury were higher than the average values for the whole group, and considerably higher than the blood levels of the nondepressed patients. Irritability was observed in 44.2% of the patients, all except 1 of the 19 being under 30 yrs old. There was general improvement in the mental states of the patients who were hospitalized. Mercury binding compounds did not seem to have a significant effect in enhancing recovery from the depressive state. The possibility of there being 2 distinct syndromes, due to organic and inorganic mercury poisoning, is discussed.