Poly trans-[(2-carboxyethyl)germasesquioxane] (IUPAC name) is the most common water-soluble organic germanium compound. This compound is known as bis(carboxyethyl)germaniumsesquioxide and it is commonly called Ge-132; it is hydrolyzed to 3-(trihydroxygermyl)propanoic acid (THGPA) in water. We have developed a method for the quantification of THGPA in rat plasma, using a novel extraction method based on a reversible chemical conversion. THGPA in plasma is converted to 3-(trichlorogermyl)propanoic acid (TCGPA) under acidic conditions using concentrated hydrochloride, which is followed by extraction with chloroform. TCGPA is then converted back to THGPA through hydrolysis. The extraction recovery of this method is approximately 100%. Moreover, we synthesized deuterated Ge-132, which was used as an internal standard in our experiments. This method covers a linearity range of 0.01-5 μg/mL for concentrations of THGPA in plasma. The intra-day and inter-day precisions of the analysis are about 4.1%, and the accuracy is within ±2.6% at THGPA concentrations of 0.025, 0.25, and 2.5 μg/mL. The total run time is 5 min. Our method was successfully applied to a pharmacokinetic investigation following oral administration of Ge-132.