FDLE cells were maintained in gas mixtures that mimicked either the fetal (PO2 = 23 mmHg) or adult alveolar (PO2 = 100 mmHg, n = 6) environments for 24 h. Experimental cells were then transferred between the two environments and, after a further 24 h, their responses to basolateral isoprenaline (10 μM) were monitored. These data are presented (means ±s.e.m.) together with the responses of age-matched control cells that had been isolated from the same litters and continuously maintained at the original PO2. A, responses of cells initially maintained at 23 mmHg (n = 7). B, responses of cells initially maintained at neonatal PO2 (100 mmHg, n = 6).