Characteristics of the fetal heart rate (FHR) were studied longitudinally throughout the second and third trimesters in 43 low-risk singleton fetuses. The second trimester was characterized by an FHR pattern showing minimal differences between quiet and active intervals of fetal behavior. Few movements were accompanied by accelerations of small magnitude. Brief decelerations of variable magnitude exceeded the number of accelerations, and their frequency declined with advancing maturity. The percentage of body movements accompanied by FHR acceleration, the amplitude of these accelerations, and the rate of rise in heart rate increased with gestational age. Up to 30 weeks, the characteristics of the baseline FHR were similar in both periods of fetal rest and activity, with the baseline variability becoming significantly greater with advancing gestation. However, during quiet cycles of fetal behavior after 30 weeks, the baseline variability became progressively narrower, so that in late pregnancy different behavioral states could be determined reliably from characteristics of the baseline FHR alone.