Short-term cultured dorsal unpaired median neurones from adult cockroach, Periplaneta americana, have been used to study alpha-bungarotoxin-resistant cholinergic receptors. Both acetylcholine and nicotine applied by pressure ejection to the neuronal soma induced depolarizing responses recorded with the patch-clamp technique in the whole cell recording configuration. Nicotine was more potent than acetylcholine and developed a dose-dependent biphasic depolarization including a fast and a slow component. The slow component was sensitive to alpha-bungarotoxin, d-tubocurarine, pirenzepine and gallamine, whereas the fast component was resistant to these nicotinic and muscarinic antagonists. These results demonstrate that two distinct functional receptors exist: a sensitive nicotinic and a 'mixed' (nicotinic muscarinic) receptor governing a nicotine-induced biphasic response.