Summary.We have identified a variety of biological rhythms involved in the apposition and mineralization of dentin in the rabbit incisor.Animals were injected during the day or night with lead acetate at 2-week intervals--to provide biological time markers in forming dentin--and transverse undecalcified sections of the lower incisors were prepared for electron microprobe analysis. The positions of the lead markers were identified, and the continuous distribution of calcium and sulfur was measured at 1 /xm intervals between the markers. In thin sections stained with hematoxylin after decalcification, the widths of a series of structural increments (bands) were measured with an ocular micrometer.Fourier analysis of the data revealed spectra of structural and compositional rhythms with a range of periodicities which extended from a matter of hours [ultradian (< 24 h)] to days [infradian (> 24 h) and circadian (approximately 24 h)].The structural and compositional rhythms appeared to be independent to the extent that they did not necessarily have the same periods, or amplitudes. Nor were there simple phase relationships between all of the rhythms. At some times, Ca and S fluctuations are inversely proportional (180 ~ out of phase), but in other cases they are directly proportional or out of phase by varying degrees other than 180 ~ . The analyses thus suggest that calcium and sulfur deposition (representing mineral and glycosaminoglycan deposition, respectively) are not simply inversely proportional, and that the hematoxylin-stained structural increments did not solely reflect differences in the distribution of the mineral components in dentin.Send ofll)rint requests to Dr. Gary D. Rosenberg, at the above address