The compounds are substituted 1,2,4-triazin-5-ones which are histamine H.sub.2 -antagonists. Two specific compounds of the present invention are 3-[2-(5-methyl-4-imidazolylmethylthio)ethylamino]-6-(3-methoxybenzyl)-1,2, 4-triazin-5-one and 3-[2-(5-methyl-4-imidazolylmethylthio)ethylamino]-6-(3-pyridylmethyl)-1,2, 4-triazin-5-one.