Previous investigations of the effect of glucocorticoids on fetal lung development have suggested that changes in lung structural properties may be of greater functional significance than changes in lung surfactant. In this study we examined the mechanism of the glucocorticoid-induced change in lung structure and compared it with the changes that occur during normal development. Rhesus monkeys (Macaca Mulatta) were treated with betamethasone prior to 133 days gestational age and then delivered by cesarian section either at 133 days or near term at 160 days. Our results show that normal fetal lung development over the last month of gestation occurs with a 45% increase in lung dry weight and a 56% increase in the maximal air volume per gram lung (Vmax/g). The immediate effect of the steroid was to cause a similar percentage increase in Vmax/g but no increase in lung weight. In fact, there were significant decreases in lung weight at both 133 and 160 days in the treated animals. In addition, we found no significant changes in the mean alveolar size either with normal development or with the steroid treatment. We thus conclude that in the rhesus monkey, the lung maturation process involves both an increase in lung mass and an increase in the number of alveoli per gram of that mass. The betamethasone treatment results in accelerated recruitment of alveoli coupled with impaired growth of lung tissue.