We used cultured rabbit tracheal epithelium to determine the effect of mammalian-derived tachykinin on airway ciliary activity and its modulation by neutral endopeptidase EC 18.104.22.168 (NEP). Neurokinin A (NKA) caused dose-dependent increases in ciliary beat frequency (CBF), as measured by a photoelectric method, with the maximal increase from the baseline 15.7 +/- 1.7% (mean +/- SEM, p less than 0.01), whereas substance P (SP) had no effect. The NKA-induced increase in CBF was not inhibited by phentolamine, propranolol, or atropine, but it was abolished by the tachykinin antagonist [D-Pro2, D-Trp7,9]SP. Pretreatment of tissue with thiorphan (10(-5) M), a NEP inhibitor, had little effect on CBF responses to NKA; however, it significantly potentiated the responses to SP (14.9 +/- 3.0%, p less than 0.01). Other peptidase inhibitors, including captopril, bestatin, and leupeptin, did not alter the tachykinin-induced CBF response, suggesting that angiotensin converting enzyme, aminopeptidases, and serine proteinases do not modulate ciliary activity in response to tachykinins. These results suggest that NKA increases CBF by acting directly on tachykinin receptors and that NEP may play a role in modulating the tachykinin-induced stimulatory effects on CBF.