ABSTRACTThe ionic efflux during a single action potential of the algaNitellopsis obtusawas recorded by measuring the increase in conductivity of the extracellular fluid. The conductivity changes are reflected as variations in absorption of a high‐frequency field applied by a specially designed probe placed in the external medium close to the alga and connected to a high‐frequency reflectometer. The combined characteristics of probe/solution and reflectometer are given. Most experiments were performed on algae in distilled water, comparative experiments with lake water having shown that this arrangement does not introduce any significant changes in the activation processes studied. The possibility of dynamic recording of ionic fluxes is illustrated by oscillograph records showing the temporal relation between the ionic concentration change and the action potential; that the method is suitable for quantitative measurements is demonstrated by inkwriter records showing the stepwise increases in concentration caused by intermittent stimulation during hour‐long experiments. Quantitative data on the increments in ionic concentration associated with a single impulse, and on the total increase resulting from a number of stimuli, are presented and compared with chemical analyses showing the ions involved to be K+and C1‐‐. Satisfactory correspondence was obtained between the values of the conductivity measurements, tentatively expressed as KC1 concentration, and the chemical analysis. The average values for the K+efflux were 2 times 103to 4 times 103pmole/cm2. impulse; preliminary studies showed the C1– efflux to be of approximately the