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1. The relative contribution of K and Cl to the total increase of membrane conductance produced by adrenaline on the smooth muscle of guinea-pig taenia coli has been investigated. Constant current pulses were applied with a pair of large external electrodes, the voltage change was recorded with an intracellular micro-electrode, and the change in the size of the electrotonic potential by adrenaline was measured.

2. Adrenaline hyperpolarized the membrane by about 8 mV with a range from 5 to 15 mV and it reduced both the size and the time course of the electrotonic potential. On the average the size was reduced to 65 ± 1·8% (n = 75) of the normal size, from which a decrease of the membrane resistance to 45 ± 2·2% was calculated, or 48% allowing for the spatial decay.

3. The magnitude of the reduction of the electrotonic potential caused by adrenaline was decreased as the concentration of K or Cl ions in the medium was reduced by substituting NaCl or sucrose for KCl or an impermeant anion, benzene sulphonate, for Cl ions. The adrenaline effect remained unaltered when the NaCl was replaced with Tris-Cl.

4. It is therefore concluded that adrenaline opens the ion pathways mainly for K and Cl.

5. On the basis of the changes in potential and in conductance caused by adrenaline, the equilibrium potential for its action was calculated as - 75 mV.

6. The ratio of the Cl component to the K component of the additional conductance increase was obtained as 0·36 using the calculated equilibrium potential and the Nernst potentials for K (-91 mV) and Cl (-31 mV).