Towards elucidation of mechanisms in initiation and termination of apnea and occurance of bradycardia polygraphic recordings were performed to 10 preterm infants (mean gestational age 28.0 / range 26.1 to 32.1 weeks) at 36, 40, 44 and 52 weeks conceptional age including EEG, submental and diaphragmatic EMG, ECG, thoracal respiratory movements, nasal airflow, tcpO2 and tcpCO2. A total of 148 apneas (>10 sec.) were recorded. 33 of them were mixed/obstructive events in N-REM-sleep and 66 in REM-sleep. Apneas followed a sigh in 16 of 106 REM apneas and 29 of 43 N-REM apneas (20 of the N-REM apneas were mixed with the inactive part proceeding the obstructive part). A significant suppression of EEG-activity (Wilcoxon Rank, p<0.05) was found during mixed/obstructive N-REM apneas and inactive REM apneas. Moreover in N-REM-sleep a more distinct EEG-suppression, which was significant for the 8-13 Hz band (Wilcoxon, p<0.05), accompanied the apneas with an initial sigh in contrast to those without. Bradycardia occured in 13 N-REM apneas and in 11 REM apneas also with a preference of mixed apneas in both sleep-phases (CHI-square, p<0.005). A parallel occurance of initial sighs and bradycardia was found in 10 N-REM mixed apneas only.