The mammalian suprachiasmatic nuclei (SCN) transmit signals to the rest of the brain, organizing circadian rhythms throughout the body. Transplants of the SCN restore circadian activity rhythms to animals whose own SCN have been ablated. The nature of the coupling signal from the grafted SCN to the host brain is not known, although it has been presumed that functional recovery requires re-establishment of appropriate synaptic connections. We have isolated SCN tissue from hamsters within a semipermeable polymeric capsule before transplantation, thereby preventing neural outgrowth but allowing diffusion of humoral signals. Here we show that the transplanted SCN, like neural pacemakers of Drosophila and silkmoths, can sustain circadian activity rhythms by means of a diffusible signal.