Abstract: A class of pyrazine, pyrimidine and pyridazine derivatives, substituted by a non-aromatic azabicyclic ring system and optionally by up to two further substituents, is of use in the preparation of medicaments, especially formulations adapted for topical administration to the eye, suitable for the treatment of glaucoma and/or for reducing intraocular pressure.

Abstract: The use of compounds for formula (I): ##STR1## or a salt or prodrug thereof; wherein one of X, Y, or Z is an oxygen or sulphur atom and the other two are nitrogen atoms, and the dotted circle represents two double bonds thus forming a 1,3,4-oxadiazole, 1,2,4-oxadiazole, 1,3,4-thiadiazole or 1,2,4-thiadiazole nucleus;R.sup.1 represents a non-aromatic azacyclic or azabicyclic ring system selected from ##STR2## wherein the broken line represents an optional chemical bond; the substituents R.sup.3 and R.sup.4 may be present at any position, including the point of attachment to the oxa- or thia-diazole ring, and independently represent hydrogen, C.sub.1-4 alkyl, halo, C.sub.1-4 alkoxy, hydroxy or carboxy, or R.sup.3 and R.sup.4 together represent carbonyl;the group R.sup.5 represents hydrogen or C.sub.1-4 alkyl; andR.sup.2 represents hydrogen, C.sub.1-8 hydrogen, C.sub.1-8 alkyl optionally substituted by hydroxy or fluoro, C.sub.2-8 alkenyl, OR.sup.7, SR.sup.7, NR.sup.7 R.sup.8, CN, CO.sub.2 R.sup.7, CONR.sup.7 R.