It has been considered that the control mechanism of FHR is regarded by its central nervous system (CNS). Our attempt were focused on the differences of FHR changes between normal fetuses and fetuses of diabetic mothers. A hundred and fifteen normal fetuses (24 to 36 gestational weeks) and 16 diabetic mothers (28 to 36 gestational weeks) were investigated by means of NST. Since acceleration associated fetal movement increased significantly from 28 to 36 gestational weeks in normal fetuses, control mechanism of FHR regulated by CNS was regarded to be established in this period. Increasement of the acceleration was also observed after 28 gestational weeks in fetuses of dia-betic mothers. The acceleration of fetuses of diabetic mothers was, however, less prominent than those of normal fetuses during 32 to 35 gestaional weeks, whereas those accelerations were observed at same extent at 36 gestational weeks. Our findings indicate that delay in maturation of CNS in fetuses of diabetic mothers occurred during 32 to 35 gestational weeks. Study on maternal factors regarding maturity in fetuses of diabetic mothers showed poor correlation between the acceleration and the levels of maternal plasma glucose and HbA1. The acceleration of heavy for date group tended to be less frequent than that of appropriate for date group in fetuses of diabetic mothers.