Sequential adhesive tape stripping was implemented to characterize the penetration of nickel salts in human stratum corneum. Exposure areas of the salts in methanol applied open on arm and back skin in low volume were stripped 20 times to the level of the glistening layer at intervals of 30 min to 24 h post-dosing, and the strips analyzed for metal content by inductively coupled plasma-atomic emission spectroscopy. In the case of nickel chloride, sulfate, nitrate and acetate, material left on the skin surface, the depth-penetration profiles in the stratum corneum, and the dosage unaccounted for suggest the following conclusions: (a) Up to 24 h, most of the nickel dose applied remains on the skin surface or is adsorbed in the uppermost layers of the stratum corneum. (b) At higher concentrations, incomplete material recovery becomes discernible; within 24 h, nickel salts thus appear to penetrate beyond the stratum corneum to a minor degree, possibly via the skin shunts. (c) While the concentration gradients of nickel adsorbed vary with counter ion, anatomical site, dose and exposure time, for all variables tested the depth profiles converge to non-detectable levels (< 20 ppb) towards the level of the glistening layer. A notable exception is nickel as nitrate, for which levels continue at low but constant levels (1% of dose) beyond the third stratum corneum strip, indicative of intercellular diffusion. (d) Differences in material recovered suggest that the stratum corneum on the arm is more penetrable to nickel than stratum corneum on the back. (e) The counter ion in nickel salts plays a major part in their diffusion into the stratum corneum, suggestive of ion pairing. Overall, the data point to all three avenues of skin penetration by nickel: intracellular, intercellular, and transappendageal.