To quantitate fetal lung cellularity and regional variation in alveolar maturation, guinea pig lungs were studied at 55, 60, and 65 d of gestation or within 2 h of birth (term = 68 d), and the data were analyzed for intralobar, interlobar, and age-group differences. Nine blocks from each left cranial and caudal lobe were all measured for volume, numerical, and surface densities in tissue (Vv(i,t), Nv(i,t), and Sv(i,t], and total volumes, numbers, and surface areas per lung (V, N, and S) of type I and type II epithelia, presumptive progenitor epithelium (cuboidal cells lacking lamellar bodies [LB]), interstitium, and endothelium. Total fixed lung volume, VL, increased 3-fold from day 55 through birth. At each age, there were no consistent intralobar or interlobar differences in Vv(i,t), Nv(i,t), or Sv(i,t) for any cell type. Within a septal tissue volume of 580 to 670 microliters at all ages, the N and V of type I cells did not vary with age, although their S increased from 1,240 cm2 at day 55 to 3,967 cm2 at birth. The N of morphologic type II cells per lung increased 7-fold from day 55 to day 60 and was constant thereafter, while the N of cuboidal cells decreased proportionally; type II cells contained only 4.8% (vol/vol) of LB at 55 d compared to 18.0% at birth. The N and V of interstitium did not vary by age. While endothelial V was constant over these ages, endothelial S increased from 897 cm2 to 3,398 cm2, and the V of capillary blood and the V of alveolar airspace each increased 4-fold.(ABSTRACT TRUNCATED AT 250 WORDS)