Although catecholamines are believed to be the primary chemosensory transmitters, acetylcholine (ACh) is also present in type I cells, and is released during stimulation of the carotid body (Eyzaguirre & Zapata, 1968; Fidone & Gonzalez, 1986). Effects of exogenous ACh vary with species (Fidone et al., 1990), but in the rat and cat, excitatory effects such as increased carotid sinus nerve activity or the stimulation of catecholamine release are observed (Shaw et al., 1989; Kholwadwala & Donnelly, 1992). These effects are mediated by nicotinic ACh receptors (nAChRs). Here, we have examined the actions of nicotinic agonists on isolated type I cells to determine whether nAChRs are present on these cells, and how their activation might lead to the reported excitation of the intact carotid body.