Nuclei from HeLa and other mammalian cells have been injected into Xenopus oocytes. The synthesis, uptake, and release of RNA and proteins by injected nuclei have been investigated by autoradiography. Injected nuclei which undergo enlargement synthesize RNA continuously for up to 28 days. When oocytes are incubated in [3H]uridine or [3H]guanosine, injected nuclei are labelled nearly as strongly as the nucleoli, but much more strongly than the nucleoplasm of the oocyte's germinal vesicle. Injected nuclei appear to increase their rate of RNA synthesis during incubation in oocytes. This apparent increase in the rate of RNA synthesis is correlated with nuclear enlargement, as well as with the loss of protein from injected nuclei and with their uptake of histone and nonhistone proteins from oocyte cytoplasm. Injected HeLa nuclei lose most of the previously synthesized RNA from their nucleoplasm, but little if any of the RNA from their remaining nucleoli.