Per-and polyfluoro alkyl substances (PFAS), also known as "forever chemicals", are deemed as highly toxic with similar toxicological mode-of-action (MoA) and potency. However, varying carbon chain length and functional head-group of PFAS can affect their physicochemical properties, resulting in different toxicological properties. To assess PFAS toxicological MoA and to distinguish between high toxic PFAS and the low-toxic analogs, we tested a set of eight PFAS with varying carbon chain length (C2-C10) in the ToxProfiler assay. ToxProfiler is a human in vitro assay containing seven fluorescent reporters to visualize and quantify activation of the major cellular stress pathways: oxidative stress, cell cycle stress, endoplasmic reticulum (ER) stress, autophagy, ion stress, protein stress and inflammation. In addition, we evaluated teratogenicity potential of long-chain PFAS perfluorooctanoic acid (PFOA
C8), and the ultrashort-chain PFAS trifluoroacetic acid (TFA
C2) in ReproTracker, a human induced pluripotent stem cell (hiPSCs)-based assay in which differentiation into cardiomyocytes, hepatocytes, and neural rosettes is followed to identify developmental toxicity hazards of new drugs and chemicals. In this study, we identified long-chain PFAS (C8-C10), such as PFOA (C8) to be more cytotoxic than ultrashort-chain PFAS and to predominantly induce ER and oxidative stress at 130 µM. PFAS with a carbon chain length of C4-C7 primarily induced autophagy (300 µM) in ToxProfiler. Ultrashort-chain PFAS trifluoroacetic acid (TFA
C2) and perfluoropropionic acid (PFPrA
C3) did not activate any of the ToxProfiler stress response reporters and were not cytotoxic at their maximum tested concentrations (10 mM). In concordance, exposure of differentiating cells to PFOA in ReproTracker led to a concentration-dependent decrease in the hepatocyte-specific and neuroectodermal biomarker genes and disrupted their morphology at 30 and 60 µM, respectively. TFA had no significant effect on biomarker expression, nor on the morphology/functionality of the three differentiated cells. Altogether, we demonstrated that the carbon chain length of PFAS can determine their in vitro toxicity and ultrashort-chain PFAS (TFA) were found to be less toxic when compared to long-chain PFAS.