Continuous measurements of human fetal breathing movements and gross fetal body movements were made with an ultrasonic real-time scanner for periods of 24 houro 31 weeks' gestation, a significant increase in fetal breathing movements occurred during the second and third hours after meals; this pattern apparently followed an increase in maternal plasma glucose concentrations. At 38 to 39 weeks' gestation, fetal breathing movements increased during the second and third hours after breakfast, but the troughs seen before lunch and supper at 30 to 31 weeks' gestation were not present. Fetal breathing movements diminished over the day and reached a minimum between 1900 and 2400 hours. Fetal breathing activity increased in both groups between 0400 and 0700 hours while mothers were asleep; this was not related to an increase in maternal glucose concentrations. Absence of fetal breathing movements was observed for up to 122 minutes in this analysis of 480 hours in 20 patients. These data show that much more information must be obtained on factors that normally influence fetal breathing activity. Only then can research strategies be suggested for clinical evaluation of the usefulness of fetal breathing movements in the assessment of fetal health.