Sildenafil has been implicated in the activation of cystic fibrosis transmembrane conductance regulator (CFTR) protein. The effect was observed in vitro with doses roughly 300 larger than those commonly used for treating erectile dysfunction. We assessed the therapeutic efficacy of a single intraperitoneal injection of two clinically approved phosphodiesterase 5 (PDE5) inhibitors, sildenafil (0.7 mg/kg) and vardenafil (0.14 mg/kg), on sodium and chloride transport by measuring the transepithelial potential difference (PD) across the nasal mucosa of F508del (van Doorninck et al, 1995), cftr knockout and normal homozygous mice. Both CF animal models showed ion transport abnormalities similar to those observed in CF patients, i.e. a large negative PD in basal conditions; an increased depolarising response to amiloride, which blocks the epithelial sodium channel ENaC; a reduced response to perfusion with chloride-free solution containing amiloride plus forskolin, a cAMP agonist. In F508del mice, but not in cftr knockout mice, the chloride conductance was corrected to the same level of that recorded in normal homozygous mice 1 h after sildenafil administration. A more prolonged effect, persisting for at least 24 h, was observed with vardenafil. The forskolin response was increased after sildenafil and vardenafil in both normal and F508del mutant animals. To determine whether the effect of PDE5 inhibitors in increasing chloride transport is dependent on alternative chloride transport pathways, we examined the effect of sildenafil in the presence of 200 mM 4-4'-diisothiocyanostilbene-2,2'-disulphonic acid (DIDS), an inhibitor of alternative chloride channels. In F508del animals, PD changes after DIDS were at least 4-fold higher in sildenafil treated than in placebo treated animals (-9.5  0.3 vs –2.0  0.0; p < 0.0001; n = 5 in each group). This finding indicates that the effect of sildenafil in F508del mice does not principally depend on alternative chloride transport pathways. No effect on the sodium conductance was detected in any group of animals. Our results provide preclinical evidence that both PDE inhibitors, applied at clinical doses, stimulate chloride transport activity of F508del-CFTR protein.