Tracheal pressure and electrocortical activity were recorded in 10 unanesthetized fetal lambs in utero to determine the effect on fetal breathing movements (FBMs) of fasting and a subsequent infusion of glucose. The incidence of FBMs decreased significantly from 37.9% +/- 4.3% during fed control studies to 23.6% +/- 3.6% (P less than 0.05) after 24 to 36 hours of maternal fasting, with a related fall in fetal blood glucose. A 2-hour infusion of glucose to the fetuses of fasted animals resulted in a significant increase in FBMs to 39.5% +/- 4.9%, but only to a level similar to that in the fed control studies despite a twofold increase in the fetal concentration of glucose above fed control values. Changes in the incidence of FBMs paralleled the changes in fetal low-voltage electrocortical activity with alterations in glucose. In conclusion, FBM, although correlated with the concentration of glucose, may be influenced more by fetal hypoglycemia than hyperglycemia. Glucose appears to affect the incidence of FBM by altering the incidence of low-voltage electrocortical activity.