Slow eye movements (SEMs) during sleep onset and their relationship to vegetative rhythms were investigated in six healthy, sleep-deprived subjects, yielding 143-289 SEMs in an epoch of 31.5-56 min per experiment. Exclusively, sleep in stages I and II was recorded. From the bandpass-filtered electro-oculogram (EOG) signal (cut-off frequencies 0.05 and 1 Hz), turning points of the gaze were detected and compared with the start of inspiration, which was discriminated from the abdominal respiratory excursions. SEM cycle times varied considerably more than respiratory cycle times (P < 0.05 in Levene's test). Both were preferentially of equal length, or, in some subjects, in 2:1 co-ordination. Cross-correlation histograms yielded that inspiration and SEMs were also temporally co-ordinated. Thus, there is a temporal coherence regarding the occurrence, the cycle time and the phase between SEMs and a respiratory-like rhythm. Our findings show that it is not exactly the respiratory rhythm that is mirrored in the SEMs. Rather, we favour the interpretation of an autorhythmicity that is temporarily connected to the common brainstem system in the reticular formation of the brainstem.