8 nickel-sensitive subjects were given a gradually increasing daily oral intake of NiSO4 in water. The exposure lasted from between 91 and 178 days and the total intake ranged from between 113 and 278 mg of Ni++. While 6 subjects were continuously exposed over the entire period, the other 2 were exposed for 2 shorter periods with an interval between the 2 exposures of 84 and 63 days, respectively. Nickel exposure was well tolerated by all subjects, and there was no worsening of the cutaneous manifestations. Ni++ serum and urine concentrations were repeatedly assayed. A reduction of intestinal adsorption and an activation of the renal excretion were shown through an evaluation of the ratios of Ni++ serum concentration/Ni++ cumulative oral intake, Ni++ urinary amount/nickel cumulative oral intake and Ni++ serum amount/Ni++ urine amount. The course of Ni++ faecal amounts, calculated indirectly, increased rapidly in time and was consistent with the other courses. In many subjects, the decrease in serum concentrations was followed by a slight increase. It is likely that this phenomenon is due to the release of epidermally stored nickel. These data seem to indicate that in some sensitive subjects, prolonged oral exposure to NiSO4 in water reduces the intestinal adsorption of nickel and activates its renal excretion, also promoting the mobilization of accumulated element.