A method of treating cystic fibrosis in a patient comprising: administering to said patient an effective amount of a type V cyclic nucleotide phosphodisterase inhibitor, wherein said inhibitor is a compound of formula (I): ##STR1## wherein A is a five- or six-membered ring containing one or more N atoms, X is a substituent of formula (II): ##STR2## wherein R.sub.1 represents C.sub.1-4 alkyl, and X is positioned at either or both of the 2-position and/or the 8-position in (I).