We have previously shown that alveoli of neonatal rat lungs contain 2 morphologically distinct fibroblasts; 1 that contains lipid droplets and 1 that does not. We have named these lipid interstitial cells (LIC) and nonlipid interstitial cells (NLIC). In this study, using stereologic methods, we evaluated the distribution of LIC and NLIC in the alveoli of immature and mature rats, hamsters, and mice and quantitated cytoplasmic lipid in the LIC. We found that fibroblasts account for approximately 50% of resident alveolar wall cells in immature and 20% in mature rodent lungs. At each age LIC accounted for half of the fibroblasts. There were age and species differences in the amount of LIC lipid. The volume density of cytoplasmic lipid was higher in immature rats (0.285) than in mice (0.205) or hamsters (0.010) and was higher in mature mice (0.176) than in hamsters (0.051) or rats (0.019). Possible explanations for these differences are discussed. Bundles of characteristic cytoplasmic myofilaments established that the LIC was identical to the previously described contractile interstitial cell of the lung. These studies confirm the presence of 2 distinct fibroblasts in the alveoli of immature and mature rodents of several species.