Failure to arouse from sleep is a possible mechanism leading to sudden infant death. Using a controlled pulsatile air jet applied alternately to the nostrils we have made multiple measures of arousal threshold both between and within sleep states. Infants (n= 22) born at term were studied at 2-3 wk postterm (mean age 13 d, range 9-17 d, study 1) and again at 2-3 mo postterm (mean age 78 d, range 56-98 d, study 2). Arousal threshold (stimulus driving pressure, cm H2O) was determined in both active sleep (AS) and quiet sleep (QS). At both ages arousal threshold in QS was significantly higher (251 ± 24 and 298 ± 35) than in AS (163 ± 19 and 144 ± 29) (p< 0.001). In a morning sleep period, the first and second QS epochs were compared in each baby. In both study 1 and study 2, respectively, arousal thresholds were significantly higher in the second QS epoch (270 ± 34 and 497 ± 100) than in the first QS epoch (198 ± 29 and 252 ± 69) (p< 0.05 andp< 0.02). There was a significant correlation in individual infants between arousal thresholds in the two states at both ages (p< 0.005 andp< 0.007, respectively). Regression analysis showed no correlation between the length of time the infant had been in a particular sleep state and the arousal threshold in either state in study 1 or in AS in study 2; however, arousal threshold increased significantly(p< 0.01) with time in QS in study 2. This study has expanded on previous findings that arousability is sleep state-dependent by demonstrating that arousability in QS is also altered by the length of time that the infant has been asleep.Abbreviations: SIDS,sudden infant death syndrome;QS,quiet sleep;AS,active sleep;HR,heart rate;SaO2,oxygen saturation