After intramuscular administration of graded doses of HI-6 (62.5, 125, 250, and 500 mg) to 22 healthy men, it has been established that therapeutic concentrations of the oxime in plasma, arbitrarily taken as 4 micrograms/ml, were achieved by doses of 250 and 500 mg in about 5 min, and maintained from 2 to 3 hr. The two lowest doses have not been satisfactory in this respect. Of the total doses injected, from 56.3 to 62% of HI-6 was excreted into urine unchanged during the first 6 hr. No side-effects were reported by the subjects, nor revealed by clinical or laboratory tests during the study. Exceptional tolerance of HI-6 in man found in this study, along with its high efficiency proven in experimental poisoning by sarin, VX, and soman, make it the most promising oxime aimed at the treatment of human poisoning by known chemical warfare nerve agents.