The raised nasal transepithelial potential difference (PD) in cystic fibrosis (CF) reflects accelerated active transport of Na⁺, and is inhibited by topical administration of the Na⁺ channel blocker, amiloride. The aim of this study was to investigate the dose-effect and time course of topically administered Na⁺ conductance inhibitors to inhibit nasal PD, including benzamil, an analog of amiloride. We measured the magnitude of drug inhibition of Na⁺ transport [percent inhibition of baseline PD ( Δ PD%)] and duration of inhibition of PD, defined as the time when drug inhibition of PD had recovered by 50% (effective time = ET₅₀). Amiloride [10^{− 3} M (n = 16), 3 × 10^{− 3} M (n = 9), 6 × 10^{− 3} M (n  = 7), 10^{− 2} M (n = 3)] or benzamil [1.7 × 10^{− 3} M (n = 7), and 7 × 10^{− 3} M (n = 5)] were administered to the nasal surface via an aerosol generated by a jet nebulizer and a nasal mask. The concentration-dependent magnitude ( Δ PD%) of inhibition was similar for amiloride and benzamil ( ∼ 67– 77%), whereas the duration of inhibition (ET₅₀) was about two-and-a-half times longer after benzamil administration as compared with equivalent concentrations of amiloride [1.6 ± 0.06 versus 4.5 ± 0.6 h (ET₅₀  ± SEM), at 6–7 × 10^{− 3} M]. In vitro studies of cultured normal nasal epithelia demonstrated directly that benzamil induced an approximately 2-fold more prolonged inhibition of active Na⁺ transport than amiloride. These data suggest aerosolized benzamil is a candidate long-duration Na⁺ channel blocker for CF.