We have tested whether two local anaesthetics, in the lumen of cat trachea in vivo, would block the stimulation of mucus secretion by parasympathetic nerves without altering its stimulation by a cholinergic drug. Lignocaine, 4.3-43 mM (0.1-1%) in Krebs-Henseleit, increased the output of mucus macromolecules. It abolished secretion in response to vagus nerve stimulation, but also diminished the effect of 5 microM pilocarpine. Tetrodotoxin, 50 microM, abolished vagal control of secretion without inhibiting pilocarpine's action or changing resting secretion. Though lignocaine blocks nerve-mediated secretion, its action is not simply on nerve conduction. In contrast, the effect of tetrodotoxin may be restricted to the blocking of nerves.