1. In cats under Dial anaesthesia, Ca(2+) was injected inside lumbosacral motoneurones, by passing currents between CaCl(2)- and KCl-containing barrels of compound micropipettes.2. There was a reduction in excitability and a fall in membrane resistance, both rapid in onset and quickly reversible.3. The minimum effective injection current was approximately 10 nA, and the effect reached a maximum with currents of approximately 30 nA. The mean slope of resistance change against injection current was -1.7%/nA (S.E. 0.35).4. The most common change in membrane potential was a hyperpolarization; but in nearly half the cases, there was no clear change or a small depolarization. A reversal level for the effect of Ca(2+) could be measured in five cells: on the average, it was 10 mV more negative than the resting potential.5. Observations on i.p.s.p.s showed that Ca(2+) probably does not alter g(Cl): it was concluded that the fall in membrane resistance caused by intracellular Ca(2+) is mainly due to an increase in g(K).6. These results confirm previous suggestions that a steep transmembrane gradient of Ca(2+) is essential for the maintenance of a low membrane conductivity, and that a rise in internal free Ca(2+) - whether due to influx or release from internal stores - may play an important role in regulating neuronal activity.