Spontaneous body movement starts at early fetal stage, at embryonic day (E) 12-15 in mice. In the present study, the movement of the head region was studied in E13-14 mice by in utero ultrasound imaging, together with the in vitro recording of underlying neural activities in the hypoglossal nerve and the ventral root of the upper cervical cord of an isolated brainstem-spinal cord preparation. The role of gamma-aminobutyric acid (GABA) in the generation of fetal movement was assessed using mice lacking GABA-synthesizing glutamate decarboxylase 67 (GAD67). At E14, mouth opening and tongue withdrawal were observed independently at frequency of 14/h each. This movement was rarely observed in the GAD67-deficient mouse. The intraventricular administration of picrotoxin or 3-mercaptopropionic acid abolished mouth opening in the wild-type mice. In a brainstem-spinal cord preparation, three types of neural discharge were recorded: mouth/tongue-moving burst, respiratory burst and irregular activity on the basis of their waveform, regularity in occurrence and concomitant muscle activity. In the GAD67-deficient mice, the occurrence of mouth/tongue-moving burst and irregular activity was inhibited to about 15 and 40% of those in the wild-type mice, respectively. Respiratory burst was slightly inhibited but the difference was not significant. Picrotoxin greatly reduced the frequency of mouth/tongue-moving burst. These results indicate that GABA is involved in rhythm generation in movement of the head region and support the hypothesis that cleft palate in the GAD67-deficient mouse is due to the impairment of mouth or tongue movement that assists palate formation.