Heart Rate (HR) was analyzed during the first six Quiet Sleep (QS)--Paradoxical Sleep (PS) cycles of the polygraphically-recorded nocturnal sleep in three groups of six infants each, aged respectively 1 to 2 weeks, 15 to 19 weeks, and 25 to 47 weeks. HR decreased with age, and was higher during PS than during QS, without difference between odd QS phases (1st, 3rd and 5th, which contain always, but only in oldest infant's group, Slow wave sleep (SWS) stage 3 and 4 episodes), and even QS phases (2nd, 4th and 6th, never containing SWS episodes). In the oldest infant's group, in odd QS phase only, HR was higher in the first half with respect to the second half. The HR modulation according to the QS-PS alternance was obvious in all infant groups. In the oldest infant's group the QS phases without SWS episodes were characterized by a dissociation between EEG activity level (close to that of PS) and HR (similar to that of QS phases with SWS); in QS phases with SWS, however, the HR was modulated according to the type of EEG activity (stage 2 vs SWS).