The postinspiratory activity of the inspiratory muscles (Pmusl) was estimated in 12 infants (2 to 4 days old) by analysis of the rise in mask pressure after airway occlusion at end inspiration. We reasoned that if at the end of inspiration Pmusl instantaneously ceased, the mask pressure would immediately increase to the relaxation pressure value corresponding to that volume. Any delay would suggest some degree of Pmusl. In 9 infants, Pmusl reached zero before the end of an average expiration (TE), although lasting for a long portion of TE (83% +/- 25 SD). We then compared the expiratory tidal flow-volume curves during resting breathing with the curves of "relaxed" expirations. In general, the tidal curve shows a linear portion that can be extrapolated to zero flow. From this extrapolation it is apparent that the end-expiratory level (FRC) is above the resting volume of the respiratory system (Vr), the FRC-Vr difference averaging 3.11 ml/kg. In 6 infants, the tidal expiratory flow-volume curve was displaced to the left of the "relaxed" curve, whereas in the remaining infants the two curves superimposed. These analyses suggest that in infants during tidal breathing (1) Pmusl can substantially contribute to the rise in FRC, and (2) the final portion of expiration is in most cases "relaxed." In some infants, however, a braking mechanism, probably of laryngeal origin, further decreases the expiratory flow and may contribute in maintaining the mean lung volume elevated.