Susceptibility of the lung to caloric restriction is age-dependent, with more permanent damage occurring during the phases of growth and differentiation. Because the guinea pig is born with more well-developed alveoli than are other rodents, the postnatal lung of this species may better resist alveolar hypoplasia than the prenatal lung. Control animals were raised from sows provided food ad libitum during and after normal gestations (66 to 68 days). Starvation groups received 50% rations of control food intakes during 1 of three 21-day periods: prenatal starvation, with sows rationed during their last trimester (Day 45 to term); neonatal starvation, with nursing sows rationed during the 21 days postpartum before weaning; weanling starvation, with animals starved from 21 to 42 days postpartum. Lungs were fixed in situ with glutaraldehyde and analyzed for pulmonary morphometrics. At the end of starvation and before refeeding, lungs of prenatal and weanling starvation groups were significantly reduced for tissue volumes, alveolar and capillary surface areas, and pulmonary diffusing capacity. Recovery with feeding was complete for most parameters in the starved weanlings by maturity, but animals starved prenatally showed residual starvation effects as adults. The neonatally starved animals showed minimal effects of starvation on lung dimensions, both acutely and as adults. Morphologically, the lungs of some prenatally starved neonates were apparently retarded, at least to the saccular phase, and correlated with significant increases in the number of stillborn litters and in neonatal mortality within hours of parturition.