Conversion of heavy-aggregate alveolar surfactant (H) to a light-aggregate, nonsurface active form (L) is believed to involve the activity of an enzyme, namely, convertase. This conversion can be reproduced in vitro by the surface-area cycling technique. The purpose of the present study was to use this technique to investigate the developmental aspects of convertase activity in fetal, newborn, and adult rabbits. H was isolated from alveolar lavage from term [31-day gestation (31d)] fetal rabbit pups, 1-, 4-, and 7-day-old newborns, and adults, and the percent conversion to L was determined. To assess lamellar bodies (LB) as a potential source of activity in this species, these structures were isolated from lung tissue of 27-day-gestation (27d) and 31d fetuses, 1-, 4-, and 7-day-old newborns, and adults and were cycled the same as for H. LB contained considerable activity at each developmental stage i.e., ~82% of a 27d LB preparation converted to L after 3 h of cycling. In the adult, this value was 78%. Very little conversion of H was obtained from fetal lung (i.e., <20% of the 31d fetal preparation converted to L), but, by postnatal day 4, this value was greatly increased (i.e., >80% conversion) and stayed elevated to adulthood. The activity for each H and LB fraction was temperature and concentration dependent and diminished with storage at 4°C. These data suggest the LB as the source of convertase activity in the rabbit and demonstrate dramatic developmental changes in this activity after release of the LB contents to the alveoli.