This invention relates to amidine derivatives which are histamine H-2 antagonists and which inhibit gastric acid secretion. According to the invention there is provided a guanidine derivative of the formula I: ##STR1## in which R.sup.1 is hydrogen or a 1-10C alkyl, 3-8C cycloalkyl, 4-14C cycloalkylalkyl, 3-6C alkenyl, 3-6C alkynyl, 1-6C alkanoyl, 6-10C aryl, 7-11C aralkyl or 7-11C aroyl radical, the latter three being optionally substituted; ring X is a heterocyclic ring as defined in the specification; A is phenylene or 5-7C cycloalkylene or a 1-8C alkylene chain into which is optionally inserted one or two groups; and R.sup.2 and R.sup.3 are a variety of radicals described in the specification: and the pharmaceutically-acceptable acid-addition salts thereof. Manufacturing processes and pharmaceutical compositions are also described.