Drug-drug interactions between orally-administered antiretroviral therapy (ART) and hormones released from an intravaginal ring (IVR) are not known. We hypothesized that efavirenz-based ART and atazanavir/ritonavir-based ART would alter plasma concentrations of vaginally-administered etonogestrel/ethinyl estradiol, yet ART concentrations would be unchanged during IVR use. Methods: We conducted a parallel, three-group, pharmacokinetic evaluation at clinical research sites in Asia, South America, sub-Saharan Africa, and the United States between December 30, 2014 and September 12, 2016 (NCT01903031). We enrolled women with HIV who were either ART-naïve (Control group; n=25), receiving efavirenz-based ART (n=25), or receiving atazanavir/ritonavir-based ART (n=24). An IVR releasing etonogestrel/ethinyl estradiol was inserted at entry. Single plasma samples for hormone concentrations were collected 7, 14, and 21 days after IVR insertion. The primary outcome was the plasma concentration of etonogestrel and ethinyl estradiol on Day 21. Etonogestrel and ethinyl estradiol concentrations were compared between each ART group and the Control group by geometric mean ratio (GMR) with 90% confidence intervals (CI) and Wilcoxon Rank-Sum test. Secondarily, efavirenz or atazanavir/ritonavir concentrations were assessed by 8-hour intensive pharmacokinetic sampling at entry before IVR insertion and before IVR removal on Day 21. Antiretroviral areas under the concentrationtime curve (AUC0-8h) were compared before and after IVR insertion by GMR (90% CI) and Wilcoxon Signed-Rank test. Findings: On Day 21 of IVR use, participants receiving efavirenz had 79% lower etonogestrel [GMR 0•21 (0•16-0•28); p<0•0001] and 59% lower ethinyl estradiol [GMR 0•41 (0•32-0•52); p<0•0001] concentrations compared to the Control group. In contrast, participants receiving atazanavir/ritonavir had 71% higher etonogestrel [GMR 1•71 (1•37-2•14); p<0•0001], yet 38% lower ethinyl estradiol [GMR 0•62 (0•49-First Name (middle initial) Last Name