The characteristic pattern of breathing for an individual when awake at rest may be due to forebrain influences upon breathing. To examine this hypothesis we have studied the breathing pattern in 18 healthy subjects during relaxed wakefulness (W) and during Stage 4 sleep (S4), when forebrain influences upon breathing are absent or minimal. Inspiratory and expiratory times, respiratory frequency, tidal volume, and ventilation were quantified noninvasively by respiratory inductance plethysmography. The stability of respiratory variables between W and S4 sleep was tested within individuals. The results show that (i) individuals breathe differently from each other when awake and when in S4 sleep; the range between individuals during sleep being as large as it is when awake; (ii) differences in breathing pattern between two S4 periods within an individual are relatively small; (iii) the characteristic breathing pattern of an individual when awake tends to be maintained in S4 sleep. This persistence of a respiratory 'personality' into S4 sleep probably indicates that there are individual differences in respiratory rhythm generation in the absence of any forebrain influences upon breathing.