Ethnopharmacological relevance: Pyrrolizidine alkaloids (PAs) are a group of phytotoxins widely present in about 3% of flowering plants. Many PA-containing herbal plants can cause liver injury. Our previous studies demonstrated that PA N-oxides are also hepatotoxic, with toxic potency much lower than the corresponding PAs, due to significant differences in their toxicokinetic fates. Aim of study: This study aimed to investigate the oral absorption of PAs and PA N-oxides for better understanding of their significant differences in toxicokinetics and toxic potency. Materials and methods: The oral absorption of PAs and PA N-oxides in rats and in rat in situ single pass intestine perfusion model was investigated. The intestinal permeability and absorption mechanisms of five pairs of PAs and PA N-oxides were evaluated by using Caco-2 monolayer model. Results: The plasma concentrations of total PAs and PA N-oxides within 0–60 min were significantly lower in rats orally treated with a PA N-oxide-containing herbal alkaloid extract than with a PA-containing herbal alkaloid extract at the same dose, indicating that the absorption of PA N-oxides was lower than that of PAs. Using the rat in situ single pass intestine perfusion model, less cumulative amounts of retrorsine N-oxide in mesenteric blood were observed compared to that of retrorsine. In Caco-2 monolayer model, all five PAs showed absorption with Papp AtoB values [(1.43–16.26) × 10−6 cm/s] higher than those of corresponding N-oxides with Papp AtoB values lower than 1.35 × 10−6 cm/s. A further mechanistic study demonstrated that except for senecionine N-oxide, retrorsine N-oxide, and lycopsamine N-oxide, all PAs and PA N-oxides investigated were absorbed via passive diffusion. While, for these 3 PA N-oxides, in addition to passive diffusion as their primary transportation, efflux transporter-mediated active transportation was also involved but to a less extent with the efflux ratio of 2.31–3.41. Furthermore, a good correlation between lipophilicity and permeability of retronecine-type PAs and their N-oxides with absorption via passive diffusion was observed, demonstrating that PAs have a better oral absorbability than that of the corresponding PA N-oxides. Conclusion: We discovered that among many contributors, the lower intestinal absorption of PA N-oxides was the initiating contributor that caused differences in toxicokinetics and toxic potency between PAs and PA N-oxides. © 2019 Elsevier B.V.