To assess maturational changes in collagen synthesis, lung tissue was obtained from healthy Macaca nemestrina monkeys at different ages, ranging from 68% of term gestation to adulthood. We hypothesized that infants delivered prematurely have a greater rate of collagen synthesis than do older animals because of their greater rate of lung growth during gestation. Secondly, we hypothesized that lung repair in infants with hyaline membrane disease (HMD) is associated with an additional increase in lung collagen synthesis rate. Therefore, lung tissue was obtained during the first week of life from monkeys delivered at 82% of term gestation, a stage at which half of them developed HMD. The rate of total protein synthesis in lung samples was determined by measuring the incorporation of [³H]proline; the rate of collagen synthesis was determined by measuring the conversion of proline into hydroxyproline. Premature monkeys had a higher rate of collagen synthesis (9.9 ± 2.7 nmol/mg DNA/h) than did term infants (5.3 ± 1.1) or older animals (2.1 ± 0.4, p < 0.05). There was no additional increase in rate of collagen synthesis in animals with HMD from 3 h (14.3 ± 6.9) to 7 days of age (15.1 ± 6.1); control premature animals also had no significant change during the first week of life (10.9 ± 3.0 at 3 h; 11.6 ± 4.6 at 7 days). The early stage of recovery from HMD in premature monkeys does not appear to be associated with an increase in collagen production beyond the already increased synthesis rate associated with lung growth.