Augmenting expiratory cells (n = 23) were recorded in the rostral medulla of five cats in sleep and wakefulness. The objective was to determine the relationship of their activity to the duration of expiration (TE) and, particularly, to TE in rapid-eye-movement (REM) sleep, when expirations are short and may even cause fractionated breathing. Correlation analysis (Kendall's ) showed no consistent relationship in any state between the breath-by-breath mean activity of augmenting expiratory cells and TE. This result contradicts predications of an inverse relationship between augmenting expiratory activity and TE. Some cells (11 of 23) were more active in REM than in non-REM sleep and were active during fractionated breathing. This suggests that fractionated breathing in REM sleep is caused by short expiratory phases and not by intermittent inhibition of an ongoing inspiration.