1 The uptake of 5-hydroxytryptamine (5-HT) and beta-phenylethylamine (PEA) and their deamination by monoamine oxidase (MAO) were studied in perfused lung from male and female rats exposed to 100% O(2) at 1 ATA for up to 60 h.2 The uptake and metabolism of 5-HT in lungs from both male and female rats was not changed by exposure to O(2).3 The uptake and metabolism of PEA by lungs from male rats was unchanged. Uptake of PEA by lungs from female rats was inhibited 20% and 62% after 37 h and 50 h exposure respectively.4 MAO activity, both in vitro and in perfused lung, was increased towards PEA after 35 h of hyperoxia.5 Metabolism of PEA in perfused lung, measured over 30 min, was inhibited 52% after 50 h of O(2) hyperoxia.6 These results show that exposure to high concentrations of O(2) damages lung, resulting in inhibition of uptake of PEA and consequently in inhibition of metabolism of PEA.7 These results also indicate that, in lung from female rats, MAO-type B is more susceptible to changes in O(2) tension than MAO type A.