The invention provides compounds of general formula ##STR1## and physiologically acceptable salts, hydrates and bioprecursors thereof, in whichY and Z, which may be the same or different, each represent oxygen, sulphur, .dbd.CHNO.sub.2 or .dbd.NR.sub.3 where R.sub.3 is hydrogen, nitro, cyano, lower alkyl, aryl, alkylsulphonyl or arylsulphonyl;p has a value from 2 to 12;R.sub.1 represents ##STR2## in which R.sub.4 and R.sub.5 which may be the same or different, each represent hydrogen, lower alkyl, cycloalkyl, lower alkenyl, aralkyl or lower alkyl interrupted by an oxygen atom or a group >N-R.sub.6 in which R.sub.6 represents hydrogen or lower alkyl, or R.sub.4 and R.sub.5 together with the nitrogen atom to which they are attached from a 5 to 7-membered saturated heterocyclic ring which may contain an additional oxygen atom or the group >NR.sub.6 ;Q represents a furan or thiophen ring in which incorporation into the rest of the molecule is through bonds at the 2 and 5 positions, or a benzene ring in which incorporation into the rest of the molecule is through bonds at the 1 and 3 or 1 and 4 positions;x represents --CH.sub.2 --, --O-- or --S--;n represents zero or 1;m represents 2, 3 or 4 andAlk represents a straight chain alkylene group of 1 to 3 carbon atoms;andR.sub.2 represents lower alkyl or the group--(CH.sub.2).sub.y E(CH.sub.2).sub.x Gin whichy represents 2, 3 or 4 or can additionally represent zero or 1 when E is a --CH.sub.2 -- group;x represents zero, 1 or 2;E represents --CH.sub.2 --, --O-- or --S--; andG represents a monocyclic 5 or 6 membered carbocyclic or heterocyclic aromatic ring; or G represents the group ##STR3## where Q' represents any of the rings defined for Q; Alk' represents any of the groups defined for Alk;and R.sub.4 ' and R.sub.5 ' which may be the same or different represent any of the groups defined for R.sub.4 or R.sub.5.The compounds of formula (I) have pharmacological activity as selective histamine H.sub.2 -antagonists.