1. Research into the pathogenesis of acute and chronic neonatal lung disease has been hampered by the lack of a suitable small-animal model of prematurity. We describe such a model that has been developed and validated in the guinea-pig. 2. Pre-term guinea-pigs delivered by Caesarian section at 65 days gestation (normal gestation 68 days) exhibited transient respiratory distress. The survival of pre-term animals was lower than that of term animals after exposure to 95% O 2 (pre-term 42% versus term 79% at 96 h, P < 0.05). 3. Pulmonary histology in pre-term animals exposed to both 21% O 2 and 95% O 2 revealed evidence of acute lung injury with atelectasis, pulmonary oedema, fibrin deposition and inflammatory cell infiltration. No evidence of lung injury was observed in term animals exposed to 21% O 2 , whereas those exposed to 95% O 2 showed a similar, but less pronounced, injury to that seen in preterm pups. 4. The protein concentration in bronchoalveolar lavage fluid was similar in pre-term and term animals exposed to 95% O 2 , but neutrophil numbers in bronchoalveolar lavage fluid tended to be greater in preterm pups. 5. Elastase-like activity, measured against succinyl-1-trialanine p -nitroanilide, was higher in bronchoalveolar lavage fluid from control pre-term animals compared with that from control term animals. Exposure to 95% O 2 increased the elastase-like activity significantly in both groups. The majority of the elastase-like activity was EDTA-sensitive and thus is possibly due to metallo-elastase. Fractionation of bronchoalveolar lavage fluid indicated that the elastase-like activity was associated with a high-molecular-mass complex. Lipase treatment reduced the activity of this fraction and generated a new 40 kDa fraction. 6. We conclude that the pre-term guinea-pig is more susceptible than the term animal to lung injury after O 2 exposure and thus represents a appropriate small-animal model in which to investigate the pathogenesis of acute and chronic lung injury in the pre-term infant.