The cyclic organization of spontaneous movement was stuied longitudinally in 22 fetuses of diabetic mothers (FDMs) during the third trimester of gestation and quantified with spectral analysis. Cyclic motility at frequencies between. 08 and 1.22 cycles/min was found in all fetuses, although it was temporarily absent in 12 of the 22 fetuses at some time between 25 and 38 weeks. While some parameters of cyclic motility changed little, the frequency of the dominant motility cycle doubled from .24 to .48 cycle/min between 28 and 36 weeks, and the relative strength of motility cycles in the band between .18 and 1.02 cycles/min decreased temporarily at 36 weeks. These developmental changes and the common absence of cyclic motility contrast sharply with previous findings in normal fetuses, and (except for its absence) are accounted for by a subgroup of FDMs whose mother's blood glucose levels were least well controlled during the third trimester of gestation. The presence of cyclic motility in all FDMs provides futher evidence that cyclic activation is a basic property of the developing central nervous system in the human, as it appears to be in other vertebrates. However, the differences between FDMs and normal fetuses suggest that the development of cyclic motility is sensitive to disturbances in the fetal metabolic environment.