Lung mechanics was studied at 50 days of age in 7 rachitic rats born from mothers deprived of vitamin D. They were compared with 7 control rats raised in the same conditions but fed a diet supplemented with vitamin D. The animals were anesthetized, tracheotomized, and paralyzed. Quasi-static pressure-volume curves of the respiratory system and of the lungs were obtained. Body weight of the rachitic rats was within the range of the control rats, but dry lung weight (LW) was significantly lower (p less than 0.01). Lung volumes in absolute terms and when normalized for LW were significantly lower than in the control rats. Chest wall compliance (Ccw) of the rachitic rats was within the range of values of the control rats, except for 2 animals with an infinite Ccw. Analysis of the pressure-volume curves of the lungs of the rachitic rats compared with those in the control animals showed a significant decrease in lung compliance (CL) and in CL/LW (p less than 0.01), indicating a decrease in lung distensibility. The more severe the rickets (according to microradiographic criteria of the tibia), the lower the CL/LW. It is speculated that decrease in lung distensibility may be related to abnormal lung growth caused by disturbed alveolar formation and lung connective tissue development. These abnormalities could be due to vitamin D deficiency acting on the growing lung, as on the growing bones, by a mechanism involving proteoglycans.