Neonatal rabbits were exposed to either normoxia (21% oxygen) or hyperoxia (. 95% oxygen) for 2-4 days, and isolated ventilated perfused lung preparations from the various animals were studied. 5-Hydroxytryptamine (5HT) uptake, perfusion pressure, alveolar lavage protein and lung tissue vitamin E concentrations were measured. There was no difference in mortality between the two groups at any time point. There was no difference in perfusion pressures at any time point. There were no differences between normoxic and hyperoxic animals in alveolar lavage protein or 5 HT uptake at 2 and 3 days. At 4 days, 5HT uptake (fractional) was lower in the hyperoxia group than in controls (0.65 +/- 0.033 v. 0.75 +/- 0.013 (mean +/- SE); p less than or equal to 0.05) and alveolar lavage protein was higher compared to normoxia (1111 +/- 415 micrograms/ml v. 481 +/- 78 micrograms/ml; p less than or equal to 0.05). Lung vitamin E concentrations were higher at 3 days in rabbits exposed to hyperoxia compared to normoxia (16.5 +/- 1.8 micrograms/gm v. 12.3 +/- 0.6 micrograms/gm; p less than or equal to 0.05). In air exposed animals there was a decrease in lung vitamin E concentration after 2 days, whereas hyperoxia exposed animals had no significant decrease in lung vitamin E concentrations from 2-4 days exposure. These studies establish that the decrease in 5HT uptake, albeit delayed compared to that described previously in adult animals, is a reasonable measure of pulmonary oxygen toxicity in newborn rabbits.