The invention relates to carbonyl derivatives which are histamine H-2 antagonists and which inhibit gastric acid secretion. According to the invention where is provided a guanidine derivative of the formula I:-
in which R' and R 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 1 and R 2 is halogen substituted, or R 2 is hydrogen and R 1 is R 5 -E-W in which W is 2-6C alkylene optionally substituted by 1 or 2 1-4C alkyls, E is O, S, SO. SO 2 or NR 6 in which R 6 is H or 1-6C alkyl, R 5 is H or 1-6C alkyl optionally substituted by 1 or 2 1-4C alkyls, or R 5 and R 6 are joined to form a pyrrolidine, piperidine, morpholine, piperazine or N-methylpiperazine ring, or R 2 is H and R 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: D is O or S; R 4 is H or 1-6C alkyl; R 3 is H or a variety of radicals described in the specification: and the pharmaceutically-acceptable acid-addition salts thereof. Manufacturing processes and pharmaceutical compositions are also described.