A simple expression has been derived to predict the rate of net K uptake into exponentially growing plants of Lemna minor. Net uptake predictions are in good quantitative agreement with measurements of ‘steady-state’ K influx, indicating that, in the ‘steady state’, K movements in the plant are essentially undirectional and that efflux is small. This close matching of inward K movement to the demands of the expanding tissue is temporarily disturbed if plants are transferred to media of different K status. Uptake rates in the ‘step-up’ are initially enhanced and then fall gradually towards a new ‘steady-state’ rate. In contrast, the ‘step-down’ causes an initial depression of uptake and then rates increase gradually towards the new ‘steady-state’ rate. It is argued that these changes in uptake rates are associated with alterations in the cytoplasmic K content.