Spontaneously occurring augmented breaths (sighs) are common in infants. The pattern of electrical activity of the inspiratory muscles of the thorax and upper airway during augmented breaths, however, has not been fully characterized in this less than fully mature age group. We therefore examined costal and crural diaphragm and external intercostal and genioglossal EMG activities during spontaneous augmented breaths (n = 46) in 10 anesthetized (1.35% halothane) 1-month-old kittens breathing room air. EMG responses were assessed by comparing the spontaneous augmented breaths (AB) to the five immediately preceding breaths (control). The peak moving time average EMG activity observed during the AB was 240 +/- 32% (mean +/- SD) of control for the costal diaphragm, 279 +/- 66% of control for the crural diaphragm, and 274 +/- 68% of control for the external intercostal muscle. The mean increase in EMG activity during the AB was not significantly different among these three muscle groups (P greater than 0.25). Genioglossal EMG activity during AB was observed in only 1 of 10 study animals. These results document that during AB in anesthetized kittens, activity of the thoracic inspiratory muscles (costal/crural diaphragm and external intercostal muscles) increase in parallel, suggesting that they are modulated in a uniform manner. The infrequent observance of genioglossal activity during AB suggests that either 1) halothane anesthesia depresses genioglossal activity more than diaphragmatic and intercostal activity during AB or 2) that genioglossal recruitment is not necessary to maintain upper airway patency during this period of heightened respiratory drive.