The effect of transforming growth factor-α (TGF-α) on alveolar liquid clearance was examined in ventilated, anesthetized rats. An isosmolar Ringer lactate solution with 10, 50, or 200 ng/ml TGF-α and 125I-labeled albumin as the alveolar protein tracer was instilled into the right lower lung lobe; the rats were studied for 1 and 4 h. Compared with control rats, addition of 50 ng/ml TGF-α to the instilled fluid increased alveolar liquid clearance by 47% over 1 h and by 66% over 4 h (P < 0.05). This increase was similar to the 50% increase in alveolar liquid clearance over 1 h in rats instilled with a β-adrenergic agonist, salmeterol (28). There was a dose-dependent effect of TGF-α (10, 50, 200 ng/ml) on alveolar liquid clearance. The combination of both TGF-α and salmeterol did not have an additive effect on alveolar liquid clearance. The TGF-α-stimulated increase in alveolar liquid clearance was inhibited by amiloride (10-4 M), indicating that the increase in clearance depended on increased Na-1 uptake across the alveolar epithelium. There was only a twofold increase in intracellular cAMP levels in isolated rat alveolar epithelial type II cells after stimulation with TGF-α. In contrast, β- adrenergic agonist treatment increased intracellular adenosine 3',5'-cyclic monophosphate (cAMP) levels more than tenfold. Genistein (10-6 M), a tyrosine protein kinase inhibitor, inhibited the TGF-α-stimulated increase in alveolar liquid clearance. In summary, TGF-α can stimulate in vivo alveolar liquid clearance at a rate similar to β-adrenergic stimulation by increasing Na+ uptake by alveolar epithelial type II cells. However, the effect may be mediated by a non-cAMP dependent mechanism. Because genistein blocked the increase in alveolar fluid clearance, the signal transduction may involve genistein-dependent phosphorylation.