The pharmacologic disposition of inosine dialdehyde (IdA) was studied in mice, rats, dogs, and monkeys. 14C-IdA was utilized to follow plasma and tissue levels and patterns of excretion. Plasma radioactivity curves were multiphasic with an initial rapid component (half-life = 12-18 mins) followed by a much slower decline (half-life = approximately 3 days). As time progressed after the injection, an increasing fraction of the total plasma radioactivity was found to be associated with plasma proteins and was trichloracetic acid (TCA) insoluble. IdA interacts extensively with proteins and small molecules both in vivo and in vitro, forming complexes that do not dissociate under physiologic conditions. The pharmacologic disposition of IdA is dependent upon the relative extent of IdA interaction with protein macromolecules versus small transportable molecules. The major route of excretion was urinary with less than 50% of the dose excreted during the first 24 hours after iv administration in all species studied. IdA was recovered in the urine as complexes with urine constituents. There was no preferential uptake of IdA by any organ. All radioactivity remaining 24 hours after IdA administration was associated with plasma and cellular proteins and was nondiayzable and TCA insoluble. IdA was not a substrate for aldehyde oxidase and did not inhibit the activity of this enzyme.