We examined the mechanisms by which nitric oxide (⋅ NO) decreased vectorial Na⁺ transport across confluent monolayers of rat alveolar type II (ATII) cells grown on permeable supports. Amiloride (10 μM) applied to the apical side of monolayers inhibited ∼90% of the equivalent (I _eq) and the short-circuit (I _sc) current, with an half-maximal inhibitory concentration (IC₅₀) of 0.85 μM, indicating that Na⁺ entry into ATII cells occurred through amiloride-sensitive Na⁺ channels. ⋅ NO generated by spermine NONOate and papa NONOate added to both sides of the monolayers decreasedI _eq and increased transepithelial resistance in a concentration-dependent fashion (IC₅₀ = 0.4 μM ⋅ NO). These changes were prevented or reversed by addition of oxyhemoglobin (50 μM). Incubation of ATII monolayers with 8-bromoguanosine 3′,5′-cyclic monophosphate (400 μM) had no effect on transepithelial Na⁺ transport. When the basolateral membranes of ATII cells were permeabilized with amphotericin B (10 μM) in the presence of a mucosal-to-serosal Na⁺ gradient (145:25 mM), ⋅ NO (generated by 100 μM papa NONOate) inhibited ∼60% of the amiloride-sensitiveI _sc. In addition, after permeabilization of the apical membranes, ⋅ NO inhibited the I _sc[a measure of Na⁺-K⁺-adenosinetriphosphatase (ATPase) activity] by ∼60%. We concluded that ⋅ NO at noncytotoxic concentrations decreased Na⁺ absorption across cultured ATII monolayers by inhibiting both the amiloride-sensitive Na⁺ channels and Na⁺-K⁺-ATPase through guanosine 3′,5′-cyclic monophosphate-independent mechanisms.