The apparent lack of general recognition that hair is an important channel for the excretion of arsenic, and the paucity of data in the literature on the arsenic content of hair in cases of poisoning, prompted the present investigation of this subject when an outbreak of arsenical poisoning occurred on a farm in Sonoma County, California. Casper1first described the presence of arsenic in the hair of a patient in whom poisoning was suspected in 1860. From then to 1915 Heffter2found only six more cases by three writers in which quantitative determinations of arsenic in the hair were made. To these cases he added one of his own and included a summary of his experimental work on dogs and rabbits.3Since 1915 only two more such cases have been reported by Willcox.4The actual amounts of arsenic found in the hair in cases in which