The electrical potential difference between the exuding sap of detopped castor oil plants and the external I mM KC1 bathing solution has been measured, together with the concentrations of potassium and chloride. A typical value for the electrical potential of the sap with respect to the external solution was —56 mV, while the sap concentrations of potassium and chloride were 8 mM and 3.5 mM respectively. The total cation concentration, made up of K, Na, Ca, and Mg, was about Io m.equiv.1.; the anion deficit is made up by SO4, NO3, and PO4. A comparison of the measured potential difference with the Nernst potentials for potassium and chloride suggests that the movement of potassium into the sap is a passive process while the movement of chloride is an active process against the electrochemical potential gradient. It is suggested that the potassium and chloride ions pass through the diffusion barrier at which active transport takes place before they exchange with the other ions which appear in the sap. On the basis of this assumption it is shown that potassium is close to passive equilibrium while chloride is accumulated against an electrochemical potential difference of 110 mV or 2, 500 cal.mole−1.