The uptake and metabolism of isosorbide dinitrate was investigated in the recirculating isolated perfused rabbit lung and in lung homogenate 9000 X g supernatant. Concentration versus time profiles from the isolated lung experiments indicate rapid metabolism of isosorbide dinitrate and corresponding increases in the metabolites 5-isosorbide mononitrate, 2-isosorbide mononitrate, and isosorbide. The data suggest that the mononitrates formed in the lung tissue were converted to isosorbide at an extraordinarily high rate. Surprisingly, the rate of appearance of completely denitrated isosorbide was greater when isosorbide dinitrate was administered to the lung than when the mononitrate metabolites of isosorbide dinitrate were administered. The results suggest rapid metabolism of a substantial portion of the mononitrates formed endogenously from isosorbide dinitrate before partitioning of mononitrates into the perfusion medium could occur. The metabolism of isosorbide dinitrate in lung homogenate 9000 X g supernatant exhibited a metabolic scheme kinetically different from the intact lung studies, as isosorbide was formed slowly from a mononitrate intermediate and not by a near-simultaneous cleavage of both nitrate ester groups. Intravascular multiple-dose studies did not demonstrate any inhibition between isosorbide dinitrate and the mononitrates.