The invention relates to nitrogen heterocycles 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 and R.sup.2, same or different, are hydrogen or 1-10C alkyl, 3-8C cycloalkyl or 4-14C cycloalkylalkyl, each alkyl, cycloalkyl or cycloalkylalkyl optionally carrying one or more F, Cl or Br atoms, provided that one of R.sup.1 and R.sup.2 is halogen substituted, or R.sup.2 is hydrogen and R.sup.1 is R.sup.3 -E-W in which W is 2-6C alkylene optionally substituted by 1 or 2 1-4C alkyls, E is O, S, SO, SO.sub.2 or NR.sup.4 in which R.sup.4 is H or 1-6C alkyl, R.sup.3 is H or 1-6C alkyl optionally substituted by 1 or 2 1-4C alkyls, or R.sup.3 R.sup.4 are joined to form a pyrrolidine, piperidine, morpholine, piperazine or N-methylpiperazine ring, or R.sup.2 is H and R.sup.1 is H, 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; ring X is a heterocyclic ring as defined in the specification; A is phenylene or 5-7C cycloalkylene, or a 1-8C alkylene into which is optionally inserted one or two groups; ring Y is a heterocyclic ring described in the specification: and the pharmaceutically-acceptable acid-addition salts thereof. Manufacturing processes and pharmaceutical compositions are also described.