In one case of repeated ingestions of arsenic over a period of one year, the value of sectional toe nail analysis was investigated. The arsenic determinations were performed by instrumental neutron activation analysis. After subdividing the nail transversely into segments of 0.5 mm length, several maxima and minima of arsenic concentrations were found. Taking the nail growth into consideration, these characteristics of curve correspond to the known dates of treatments and of dismissals from the hospital. The results exclude an external arsenic contamination of the nail.