Circadian rhythms in mammals are generated by pacemaker cells located in the suprachiasmatic nucleus (SCN) of the anterior hypothalamus. The identity of these cells, however, is not known, and little information exists regarding the mechanisms by which they communicate with each other and with the organism. Nonetheless, pacemaker interactions must occur to produce single, coherent rhythms of behavior and physiology. Recently it has become possible to observe the result of these interactions using circadian chimeras, animals with two clocks with distinct periods, that have been produced by SCN transplantation. Using the tau mutation in golden hamsters, chimeras expressing two circadian rhythms of behavior simultaneously were created. The two rhythms exhibited complex interactions including cases of relative coordination. This basic result indicates that pacemaker interactions are rhythmic and phase dependent. Further analysis should help to elucidate the nature of the coupling signal and the identity of the pacemaker cells.