The compounds are substituted isocytosines which are histamine H.sub.2 -antagonists. Two specific compounds of the present invention are 2-\x9b2-(5-methyl-4-imidazolylmethylthio)-ethylamino!-5-(3-methoxybenzyl)-4-p\n' yrimidone and 2-\x9b2-(5-methyl-4-imidazolylmethylthio)ethylamino!-5-benzyloxy-4-pyrimidone\n' .