In some congenital and acquired conditions, aminoaciduria is associated with defective absorption by the mucosa of the small intestine. There is generalised aminoaciduria in nutritional rickets; thus, jejunal function was investigated. Epithelium from six patients was incubated with a dilute solution of radioactively labelled lysine and alanine. The concentration in the tissue, compared with that in the medium, was a measure of the uptake which, in turn, was thought to parallel the absorptive capacity of the gut. In the conditions of these observations, no impairment of uptake was demonstrated. © 1973 by S. Karger AG, Basel.