The present study focussed on the development and interaction of reaching and posture in supine position in young infants. The kinematics of goal directed reaches and the concurrent ground reaction forces of the total body centre of pressure (COP) in cranial-caudal and medial-lateral direction were recorded in twelve healthy fullterm infants at 4 and 6 months of age. From eight infants, data were available at both ages. The results indicated that between 4 and 6 months, postural stability during reaching in supine position increased, as the oscillatory COP displacement in the medial-lateral direction decreased significantly with increasing age. Concurrently, reaching performance improved, which was reflected by a decrease in the number of movement units (MU), the length of the displacement-path of the hand and an increase of the length and duration of the first MU. The kinematic data of the reaching movements and the COP data were correlated at both ages, but the nature of the relation changed. At 4 months of age, the number of MU was related to the number of COP oscillations in the cranial-caudal direction, whereas at 6 months the maximum velocity of the hand movement was related to the maximum velocity of COP displacement in cranial-caudal and medial-lateral direction. This change might point to the development of a more subtle control of postural adjustments during reaching movements in supine position.