Abstract The suprachiasmatic nucleus (SCN) of the hypothalamus is the site of pacemaker cells that generate circadian rhythmicity in mammals. Transplantation of the nucleus into animals whose own nucleus has been ablated results in the restoration of overt rhythmicity to the arrhythmic host. By using donors and hosts with genetically different circadian characteristics, the unambiguous recognition of the donor rhythm expressed in a transplant recipient is possible. The reappearance of a rhythm indicates that not only has the grafted tissue survived the transplantation procedure, but that pacemaker cells that generate circadian rhythms were included in the graft; this is essential in interpreting results of such transplantation experiments. The restoration of circadian function by neural transplantation has become an important tool for studying the generation and expression of biological rhythms in mammals, and is being used in the investigation of basic questions in this field.