A process for production of 2-pyrazolin-5-ones having the formula (I): ##STR1## which comprises cyclizing hydrazone derivatives having the formula (II): ##STR2## in the presence of a base; wherein R.sup.1, R.sup.2, R.sup.3 and R.sup.4 each represents a specific group; compounds having the formula (I) as well as compounds having the formula (II) and a process for production thereof.