This paper reports the first data of a study on heart rate variations of 13 5‐mo‐old infants seated in special chairs. Their electrocardiograms and behaviour were continuously recorded during sessions that were set up with their mother and a mother‐child dyad that they met for the first time when the first session was organized. The protocol comprised two interaction phases between the infants and their mothers and two with one another. The same pairs of mother‐child dyads were studied a second time 1 wk later. During the first session, the infants' heart rate variability increased significantly from the mother‐infant interaction phases to the infant‐infant interaction phases. During the second session, the variability was higher during the mother‐infant phases than in the corresponding phases of the first session. This could be linked to changes in the mothers' behaviour and the infants' idea and excitement of meeting the child from the first session again. However, the variability was significantly lower during the infant‐infant phases than during the mother‐infant phases and the corresponding infant‐infant phases of the first session. This corresponds to the development of affiliative behaviours in the infants. Conclusion: When two 5‐mo‐old infants were seated by their mothers in special chairs, significant differences were observed in their heart rate variability when they became familiar with each other, whether they were interacting with their mothers or with one another.