Abstract Levorphanol, a synthetic morphine, is known as a potent inhibitor for bacterial ribosomal RNA. Spermidine has an antagonistic effect on this inhibition. The authors applied Levorphanol on the RNA synthesis in liver cells. Levorphanol specifically inhibits the nucleolar ribosomal RNA synthesis. It does not show any effect on messenger and transfer RNA nor on other nucleolar RNA synthesis. A possible regulation of ribosomal RNA synthesis by amine oxidase is discussed in relation to spermidine.