The present invention provides pyridin-oxadiazole/thiadiazole derivatives of Formula I wherein A is N or CR 4a; and (ii) or (iii) which restore or enhance the function of mutant and/or wild type CFTR to treat cystic fibrosis, primary ciliary dyskinesia, chronic bronchitis, chronic obstructive pulmonary disease, asthma, respiratory tract infections, lung carcinoma, xerostomia and keratoconjunctivitis sire, or constipation (IBS, IBD, opioid induced). Pharmaceutical compositions comprising such derivatives are also encompassed.