Amino-acid uptake by barley leaf strips was investigated with the aid of an unnatural analogue, α-aminoisobutyric acid (AIB). Negligible metabolism of this acid by the tissue was detected. Accumulatory capacity rose with ‘ageing’ of tissue in 10−4 M CaSO4. This effect was not attributable to bacterial contamination. Net uptake was linear with time for at least 5 h and proceeded against a strong concentration gradient. The rate constant for the loss of previously accumulated labelled AIB into water was one-tenth of that into unlabelled AIB. When influx and efflux were estimated separately, ‘ageing’ was found to have reduced the rate of exit by a factor of 4. The rise in influx, though proportionately smaller, was larger in absolute value.