Autonomic and behavioral response to fear stimulation (sudden noise 80 dB) was studied in 12 sleeping infants at ages 8-50 weeks. The aim of the present study was to identify a possible passive defense response in infants. The response, which is widespread in birds and mammals, is characterized by apnea and bradycardia with circulatory changes as seen during the forced diving response. Upon stimulation, two respiratory responses were elicited: apnea preceded by irregular respiration or simple irregular respiration. Apnea was elicited in 58% of stimulations at ages 8-16 weeks compared to 14% at 28-50 weeks. The mean duration of apnea decreased from 7.8 s (+/- 1.8 s) at 8-13 weeks to 4.7 s (+/- 1.1 s) at 17-20 weeks. The preceding irregular respiration increased from 5.3 s (+/- 4.4 s) to 10.6 s (+/- 5.4 s) at the same ages. The heart rate response was biphasic and were interpreted as the orienting response. The mean deceleration in relation to apnea was 16% at 8-16 weeks and was reduced to 8% at 28-50 weeks. Infants of smoking mothers were more prone to respond with apnea than infants of non-smoking mothers (73% versus 38%). REM sleep and long postprandial sleep time increased the probability of apnea response (62% versus 38% and 66% versus 35%). The responses seen may be interpreted as expressions of the passive defense response.