The distribution of 35-S-labelled cysteine and methionine in the epidermis of the equine hoof following 2 hours of intra-arterial injection was studied by microautoradiography. Material for autoradiography was obtained by biopsy about 1 hour after termination of the intra-arterial injection and also 10 and 40 days later. In the specimens obtained one hour after the injection of labelled cysteine and methionine, the amount of radioactivity in the matrix and in the most proximal part of the laminar layer was very high. There was a clear difference between the distribution of the two labelled amino acids in the keratinizing epidermis of the hoof. Cystine was located mainly in keratinocytes of the keratogenous zone in the matrix and in the nucleated keratinocytes that formed the incompletely keratinized basal part of the primary epidermal laminae and covered the lateral surface of the outer, fully keratinized part of those laminae. Methionine was located mainly in the stratum basale and in the stratum spinosum of the matrix and in the secondary epidermal laminae of the laminar layer. In the specimens obtained 10 days after injection of labelled cysteine a considerable number of keratinocytes, both in the matrix and in the laminar layer, had attained terminal differentiation with residual labelling. Neither in the cysteine specimens obtained after 40 days or in the methionine specimen obtained after 10 days was radioactivity significantly above the basal level observed in the hoof epidermis. The possible importance of the results for research on the pathogenesis of laminitis and on the growth of the hoof is discussed.