Effects of morphine and of the protease inhibitor antipaine on RNA synthesis in nerve cells and hepatocytes of 1-, 6-, and 12-month WAG and F344 rats is studied. Slices of sensorimotor cortex, hypothalamic ventromedial nucleus, blue spot, superior cervical sympathetic ganglion, and liver were incubated with the drugs and³H-uridinein vitro. Control and experimental levels of transcription differed in examined structures and changed in the course of postnatal ontogenesis.