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The luteal phase of the female menstrual cycle is associated with both 1) elevated serum progesterone (P4) and estradiol (E2), and 2) reduced insulin sensitivity. Recently, we demonstrated a link between skeletal muscle mitochondrial H₂O₂ emission (mE_H2O2) and insulin resistance. To determine whether serum levels of P4 and/or E₂ are related to mitochondrial function, mE_H2O2 and respiratory O₂ flux (Jo₂) were measured in permeabilized myofibers from insulin-sensitive (IS, n = 24) and -resistant (IR, n = 8) nonmenopausal women (IR = HOMA-IR > 3.6). Succinate-supported mE_H2O2 was more than 50% greater in the IR vs. IS women (P < 0.05). Interestingly, serum P4 correlated positively with succinate-supported mE_H2O2 (r = 0. 53, P < 0.01). To determine whether P4 or E2 directly affect mitochondrial function, saponin-permeabilized vastus lateralis myofibers biopsied from five nonmenopausal women in …