The purposes of this study are to look at what effect multiple doses of rifampicin have on a single dose of baricitinib and to look at the safety and tolerability of these drugs. Side effects will be documented. The study will last approximately 31 days from the first dose to the end of the study.

Inclusion Criteria:

• Are overtly healthy males or females
• Male participants: Agree to use 2 reliable methods of birth control with female partners of childbearing potential during the study and for at least 3 months following the last dose of study drug
• Female participants: Women not of childbearing potential due to surgical sterilization (at least 3 months after surgical hysterectomy, bilateral oophorectomy with or without hysterectomy, or bilateral tubal occlusion/ligation) confirmed by medical history, or menopause. Menopausal women are women with spontaneous amenorrhea for at least 12 months, not induced by a medical condition such as anorexia nervosa and not taking medications during the amenorrhea that induced the amenorrhea (for example, oral contraceptives, hormones, gonadotropin releasing hormone, anti-estrogens, selective estrogen receptor modulators, or chemotherapy). Menopausal status should be confirmed by a follicle-stimulating hormone (FSH) level greater than 40 international units per liter (IU/L) at screening [unless the participant is taking hormone replacement therapy (HRT)]
• Have a body weight of ≥60 kilograms (kg) at the time of screening
• Have clinical laboratory test results within normal reference range
• Have normal renal function
• Have normal blood pressure and pulse rate (supine position)
• Have venous access sufficient to allow for blood sampling

Exclusion Criteria:

• Are currently enrolled in, have completed, or discontinued within the last 90 days from a clinical trial involving a study drug, or are concurrently enrolled in any other type of medical research judged not to be scientifically or medically compatible with this study
• Have previously completed or withdrawn from this study or any other study investigating baricitinib, and have previously received the study drug
• Have known allergies to baricitinib, rifampicin, related compounds, or any components of the baricitinib or rifampicin formulations, or history of significant atopy
• Have a history of or current cardiovascular, respiratory, hepatic, renal, gastrointestinal, endocrine, hematological, or neurological disorders
• Have an average weekly alcohol intake that exceeds 28 units per week (males) and 21 units per week (females), or are unwilling to stop alcohol consumption 48 hours prior to the first dose and until completion of the safety follow-up assessment [Day 18 ± 1; 1 unit = 12 ounces (oz) or 360 milliliters (mL) of beer; 5 oz or 150 mL of wine; 1.5 oz or 45 mL of distilled spirits]
• Have a history of, in the opinion of the investigator, excessive methylxanthine use within previous 6 months, such as greater than (>)6 cups of coffee (or equivalent) per day
• Currently smoke more than 10 cigarettes per day or are unable to abide by Clinical Research Unit (CRU) restrictions
• Are unwilling to refrain from using soft contact lenses from the start of the second treatment period until after the final follow-up

All enrolled participants.

Baseline through study completion (up to Day 32)