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Six new sesquiterpenoids, aristoyunnolins A–F (1–6), an artifact of isolation [7-O-ethyl madolin W (7)], and 12 known analogues were isolated from stems of Aristolochia yunnanensis. The structures were determined by combined chemical and spectral methods, and the absolute configurations of compounds 2, 3, 5–7, 9, 14, and 17were determined by the modified Mosher’s method and CD analysis. Compounds 1–19were screened using a bioassay system designed to evaluate the effect on mitogen-activated protein kinases (MAPKs) signaling pathways. Among three MAPKs (ERK1/2, JNK, and p38), compounds 1, 4, 10–13, 16, 18, and 19exhibited selective inhibition of the phosphorylation of ERK1/2. Compounds 16and 19were more active than the positive control PD98059, a known inhibitor of the ERK1/2 signaling pathway.