A herbicidal or plant growth-regulating 6-(pent-3-yl)-1,2,4-triazin-5(4H)-one of the formula ##STR1## in which R.sup.1 represents amino and R.sup.2 represents alkylthio having more than 1 carbon atom, alkylamino or dialkylamino, or R.sup.1 represents methylamino and R.sup.2 represents alkylthio.