Radioactive sulphur could be detected rat and mouse follicles sooner after injection than in the lamb, and this may be associated with the faster growth rate of hair in the rat and mouse. It was not possible to detect copper in follicles, although zinc was detected and found to be concentrated in the unkeratinised part of the fibre. Potassium was found in the outer sheath and in the unkeratinised part of the fibre, as well as in the stratum corneum. Labelled phosphorus was detected mainly in the bulb, with smaller amounts in the unkeratinised part of the fibre.