The nickel (Ni) concentrations of blood plasma, urine and scalp hair do not differ between hypersensitive and non-hypersensitive subjects. Those who are occupationally exposed have about ten times the control content of nickel in the plasma, urine and hair, whether hypersensitive or not. During vacations the Ni contents of their plasmas and urines decrease sharply. Oral administration of only 5 mg Ni (as the sulphate) increases the Ni concentration of plasma and urine to ten times the normal for more than the subsequent 24 h.