The electrical properties of motoneurone cell bodies in the metathoracic ganglion of the cockroach, Periplaneta americana, have been studied by the voltage-clamp technique. Most experiments were carried out on a single identified cell (cell 28), the cell body of which, as for most other insect motoneurones, is electrically inexcitable. For comparison, some experiments were also carried out on dorsal unpaired median (d.u.m.) cells, the cell bodies of which are excitable. The two cell types differed only in that the d.u.m. cells developed a transient net inward current when depolarized towards zero membrane potential. This current was reduced but not abolished by Ca-free saline. Both cell types had an N-shaped current-voltage relation, as typically seen for molluscan and other neurones, but the location of the falling phase of the relation showed an unusually strong time dependence. The N-shape was abolished by prolonged exposure to Ca-free saline, suggesting it to be due to a K conductance that was activated by the entry of Ca ions through voltage-dependent channels. An outward current was also elicited by ionophoretic injection of Ca ions. The reversal potential of this current varied with the saline K concentration, in the manner expected if the current was carried by K ions. The Ca-mediated K current was blocked by La ions and by the organic Ca antagonist D-600. A series of double-pulse experiments on voltage-clamped cell bodies of cell 28 suggested that very short periods of Ca entry were sufficient to activate the K conductance fully. These experiments also suggested that the K conductance activation was due to intracellular Ca accumulation rather than to its being directly linked to the inward Ca current. The activation of the K conductance by intracellular Ca ions was made more effective by cell membrane depolarization. The Ca-mediated conductance did not inactivate significantly under conditions in which substantial inactivation has been observed in other neurones. The physiological significance of the electrical properties of cell 28 is discussed.