The paper describes the importance of experiences of shared rhythmicity in the mother-infant relationship and their meaning for primary mental development. Rhythmical elements are already present throughout prenatal life and research has shown responsiveness of the foetus to modifications in the area of auditory perception. The author's concept of a prenatal 'sound object', which results from the foetus's experience of the maternal voice, includes rhythmical elements. Premature birth brings about a disruption of all apsects of prenatal experience including rhythmicity. Two infant observations of preterm infants from birth to one year of age show how deeply the rhythmical aspects of existence and their expression in the primary relationship can be affected, and how these children struggle to restore rhythmical aspects of experience.