Ventilation (V) decreases during sleep while upper airway resistance (UAR) increases. A number of studies have suggested that in normal healthy individuals the changes in the two variables are reciprocal. Other findings, however, suggest that the relationship between V and UAR may change as non-rapid-eye-movement (NREM) sleep progresses such that most of the change in V occurs early during the sleep period, whereas the most marked changes in UAR occur later during established NREM sleep. However, no study has examined the progressive development of changes in both V and UAR over the NREM sleep period. This study examined V and UAR over one NREM sleep period in two groups of healthy young male subjects: a "slow-wave sleep (SWS) group" (n = 8) in which the subjects obtained the full range of NREM sleep stages from wakefulness to stage 4 NREM sleep and a "no-SWS group" (n = 5) in which the subjects did not attain SWS but spent a prolonged period in stage 2 NREM sleep that was repeatedly interrupted by arousals. Results showed that the most marked changes in V occurred early during the sleep period in association with relatively small increases in UAR. Once NREM sleep became established, further attenuation of V was minimal despite marked and progressive increases in UAR. The progressive increase in UAR occurred in association with increasing delta (0.4- to 3.0-Hz) electroencephalographic activity and did not occur in the no-SWS group. We interpret these findings to indicate that factors in addition to UAR contribute to the reduction in V early in sleep onset, whereas later, during NREM sleep, compensatory mechanisms are activated to allow for maintenance of V in the context of larger increases in UAR.