The present invention provides compounds of the general formula (I) ##STR1## and physiologically acceptable salts and bioprecursors thereof in which ##STR2## represents either ##STR3## R.sup.1 represents hydrogen, halogen, C.sub.1-4 alkoxy, hydroxy, alkyl, R.sup.4 CH(OH)--, cyano or R.sup.5 CONH--, R.sup.2 and R.sup.3, which may be the same or different, each represents a straight or branched chain alkyl group or alkenyl group or R.sup.2 and R.sup.3 may, together with the nitrogen atom to which they are attached, form a saturated heterocyclic ring containing from 5 to 7 members which may optionally contain an oxygen atom; R.sup.4 represents hydrogen or alkyl; and R.sup.5 represents hydrogen, alkyl or C.sub.1-4 alkoxy. The compounds have been shown to exhibit H.sub.1 -antagonist activity in standard pharmacological tests.