In the developing fetal lung of both humans and rhesus macaques, the amount of interstitial tissue decreases during the transition from the canalicular to saccular stage. We hypothesize that this change corresponds to a decrease in restricted 1H diffusion in fetal lungs. 17 rhesus fetal lungs (in-vivo and ex-vivo) were imaged at gestation days 83-85, 110, and 133-135 with diffusion-weighted MRI. The apparent diffusion coefficients (ADCs, normalized by free-diffusion) significantly increased with gestational age for both in-vivo and ex-vivo experiments. These results demonstrate that ADC in the fetal lung can be used as a biomarker for the degree of alveolarization.