1. 1. The ionic currents of identified giant neurones of Lymnaea staynalis were measured under voltage clamp conditions. 2. 2. An essential variability was observed among the currents characterising the different neurones. 3. 3. The maximum inward current ranged between 45–113 nA, its equilibrium potential between 15 and 43 mV. The time constant of inactivation of the delayed outward current ranged between 196 and 873 msec and consisted of two components in the case of some cells. 4. 4. Following conditioning hyperpolarization on the driven neurones a fast transient outward current was activated. 5. 5. The inward channel of the examined cells may be considered to be mixed (Na +−Ca 2 +) channel. 6. 6. The differences observed in the ionic currents may be connected with the cell function.