The invention relates to novel 5-acylaminopyrimidines of the general formula I ##STR1## wherein R.sub.1 is lower alkyl, lower alkoxy, lower alkoxy-lower alkyl, thioalkyl or halogen; phenyl or phenoxy, each unsubstituted or substituted by halogen, halo-lower alkyl, carbalkoxy-lower alkyl, halo-lower alkoxy, lower alkoxy, phenoxy, halophenoxy, halo-lower alkylthio, cyano or nitro; lower alkenyl or lower alkynyl, or unsubstituted or substituted cycloalkylR.sub.2 is lower alkyl which is unsubstituted or substituted by halogen, cyano or lower alkoxy; phenyl or phenyl-lower alkyl, each unsubstituted or substituted; pyridyl, thienyl or furyl, in which the nucleus is unsubstituted or substituted, or is pyranyl, dihydropyranyl, C.sub.3 -C.sub.7 cycloalkyl or C.sub.3 -C.sub.7 alkenyl, andR.sub.3 is hydrogen or C.sub.1 -C.sub.8 alkyl and R.sub.2 and R.sub.3, together with the carbon atom to which they are attached, may form a cycloalkyl radical, which compounds are effective as plant growth regulators. The invention relates further to compositions which contain at least one of these compounds as active ingredient, and to the use thereof, as well as to the corresponding novel 5-aminopyrimidines and, as intermediates, to the novel azomethines from which they are obtained.