The hydroxypyridinonate ligands 5-LIO(Me-3,2-HOPO) and 3,4,3-LI(1,2-HOPO) are two lead compounds under development for actinide chelation therapy. Methods to quantify these actinide decorporation agents in plasma are necessary to study their in vivo pharmacokinetic behavior. Such bioanalytical methods were developed with rat plasma, using liquid chromatography coupled with tandem mass spectrometry, and have a detection range of 0.05 to 2.5 μg/mL and 0.1 to 5 μg/mL for 5-LIO(Me-3,2-HOPO) and 3,4,3-LI(1,2-HOPO), respectively. These methods were used to determine the in vivo plasma pharmacokinetics of the free acid and four salt forms of each ligand after a single intravenous or oral administration in rats. The different salt forms displayed similar pharmacokinetic profiles to those of the corresponding free acid, and the use of salt co-formers did not improve the oral bioavailability of the active pharmaceutical ingredients in rats. The described bioanalytical detection methods were successfully applied to the selection of solid-state forms of 5â??LIO(Me-3,2-HOPO) and 3,4,3-LI(1,2-HOPO) for future preclinical development activities, and will be adapted for use with plasma from other species.