An isotope procedure for the tracing of ion fluxes and rate constants in intact plants is presented and applied to 42K-labelled potassium fluxes in cells of intact barley (Hordeum vulgare L.) roots. This procedure differs from conventional tracer efflux protocols in that tracer accrual in the external solution bathing the labelled roots is continually monitored by solution subsampling, whereas conventional protocols involve monitoring the specific-activity decline in a sequence of eluates that wash out tracer released by roots. The new technique minimizes physical disturbance to the plant system, while permitting excellent time resolution of efflux kinetics. In the high-affinity transport (HATS) range, the flux and exchange parameters determined using this method showed close agreement with those found using a conventional protocol. However, in the low-affinity transport (LATS) range, substantially higher influx and efflux were seen than are normally observed with conventional tracer techniques. It is shown that this difference is attributable to the greater disturbance-sensitivity of LATS transport, and conclude that the measurement of fluxes is much more difficult in this transport range than in the disturbance-resistant HATS range.