As the shape, compliance, and deformability of the rib cage (RC) change during infancy, RC participation in quiet breathing may increase. We used respiratory inductive plethysmography (RIP) to determine the relative contributions of the RC and abdomen (AB) to tidal volume (VT) in 20 studies in 14 healthy infants 1 to 26 months of age during quiet natural sleep. RIP was calibrated with simultaneous flow measurements (anesthesia mask and pneumotachograph) by the least squares method of statistical analysis. We analyzed segments of breathing with and without flow measurement for RIP-derived VT, change in RC volume (Vrc) and AB volume (Vab) with each breath and the RC contribution to tidal breathing (%RC = Vrc/Vrc + Vab). The %RC increased with age: %RC = 1.4 age (months) + 33 (r = 0.69, p less than 0.01). After 9 months of age, %RC resembled that found in quietly sleeping adolescents. Mask placement increased VT in all but one subject (mean increase, 29 +/- 23% of baseline +/- SD; p less than 0.001, paired t test). In infants younger than 10 months of age, mask placement also increased %RC (without mask, 40 +/- 9%; with mask, 46 +/- 10% p less than 0.02). We conclude that by 1 yr of age, the RC contribution to tidal breathing during quiet sleep is similar to that of the adolescent, suggesting that major developmental changes in RC shape, compliance, and deformability take place during infancy.