Pharmacokinetic (PK) data provide pivotal information in drug development, and they areusually first studied in the preclinical stage using various animals. However, quite often, animal PKdata may not match with human PK, especially in metabolites. Thus, most regulatory agencies in theworld make it mandatory to obtain metabolite information using 14C radiolabeled drug in human forsmall molecule drug candidates. However, such studies are expensive and time consuming and theyare usually done at the end of Phase II trials using ~3.7 MBq of 14C labeled drug in a limited numberof human subjects. Introduction of accelerator mass spectrometry (AMS) in this kind of study hasrevolutionized it. Since AMS can measure 14C level as close as natural abundance, it can quantify theamounts of 14C labeled drugs and their metabolites produced in human body that consumes less thanthe amount of 0.0037 MBq of 14C labeled drug, a very safe level of radioactive dose in human. Therefore,it is now possible to conduct human 14C studies safely in early clinical trials without spending heftyamount of money and time. Korea Radioisotope Center for Pharmaceuticals (KRICP) at Korea Institute ofBiological and Medical Sciences (KIRAMS) has established an AMS facility in 2018, housing a 0.5 MV AMSmanufactured at the US National Electrostatics Corps (NEC). The AMS instrument has been validatedusing various standard samples that have been prepared at Lawrence Livermore National Laboratory inthe US, a worldly reputable provider of AMS standards. In this paper, we present a mass balance study foracetaminophen in rats using AMS and prove that the study results are equivalent with those of literature,which shows the AMS facilities at KRICP has successfully installed and be ready to be used in the variousPK studies using 14C labelled compounds for new drug development.