A new model has been established for the perfused in situ rat lung. During perfusion with glucose at 4 mmol/l, physiological concentrations of insulin (50 mu/l) caused an initial 1.5 fold increase in lactate production; the increase being sustained at 30% for the 4 h of perfusion. The observation that the lung responds to physiological concentrations of insulin could have important implications with respect to whole body carbohydrate metabolism.