The effect of the lung on insulin and glucagon were studied by comparing aortic and mixed venous levels in ambulant dogs with indwelling catheters. In the resting state ther was no difference for either hormone. After injection of insulin aortic levels exceeded mixed venous for 10 min; mixed venous levels exceeded aortic for the next 36 min. After injection of glucagon mixed venous levels consistently exceeded aortic. Single-pass studied showed an apparent gain in immunoreactive insulin as compared with [14C]inulin and 125I-albumin during passage through the lung; but no effect on glucagon could be demonstrated. A loss of immunoreactive hormone across the lung might be explained by degradation within the capillary lumen be endothelial cell surface peptidases or uptake onto specific receptors. The apparent gain of immunoreactive insulin across the lung might be due to displacement of immunoreactive exogenous hormone competing for receptor sites or modification of exogenous hormone by the lungs with increase in immunoreactivity.