ABSTRACTHydra littoralis and H. attenuata were labelled with 45Ca, 32PO4, or [3H]thymidine, either by exposing them to a medium containing the isotope or by injecting the isotope into their gut cavities. Various portions of labelled and unlabelled hydra were grafted together in diverse combinations, and the rate of transfer of radioactivity to the unlabelled portion was assessed. The relative transfer rates of the isotopes were 32PO4 > 45Ca2+ ⪢ [3H]thymidine. The transfer rates for 32P-labelled compounds and 45Ca were unaffected either by the polarity gradient (from head to foot and vice versa) or by the existence of potentially separate biological fields. Isotopes were not readily transferred between hydra or pieces of hydra which were in apposition but not grafted together. A rapid and sensitive method for the measurement of hydra mass is also described.