The cholinoceptive properties of dorsal horn neurons (lamina III-V) were investigated by means of intracellular recordings from the rat isolated spinal cord slice preparation. In half of the neurons investigated, acetylcholine (ACh) evoked a dose-dependent slow depolarization and increase in excitability; hyperpolarization was observed in 10% of neurons. Acetyl-beta-methylcholine (MCh) similarly depolarized 39% and hyperpolarized 25% of neurons tested; depolarization was also observed following bethanechol. Responses to the muscarinic agonists were abolished by atropine (10(-5) M). Nicotine depolarized 84% of tested neurons; dihydro-beta-erythroidine (5 x 10(-5) M) and (+)-tubocurarine (10(-6) M) antagonized this depolarization. ACh-, MCh- and nicotine-induced depolarizations, associated with changes in input resistance, were maintained in the presence of tetrodotoxin (10(-6) M). Substance P, as well as repetitive electrical stimulation of the dorsal root, also evoked depolarization in ACh-sensitive neurons. Atropine, but not (+)-tubocurarine, diminished responses to both substance P and dorsal root stimulation. These results indicate that dorsal horn neurons are ACh-sensitive and possess both muscarinic and nicotinic receptors. In addition, the parallel sensitivity of neurons to muscarinic agonists, substance P and dorsal root stimulation, as well as the parallel antagonistic effect of atropine, are supportive of a common ionic mechanism underlying the activation of muscarinic and substance P receptors.