There is an unmet need for an intravenous (i.v.) neuraminidase inhibitor, particularly for patients with severe influenza who cannot take oral medication. Two phase I pharmacokinetic and safety studies of i.v. oseltamivir were carried out in healthy volunteers. The first was an open-label, randomized, four-period, two-sequence, single-dose trial of 100 mg, 200 mg, and 400 mg oseltamivir i.v. over 2 h and a 75-mg oral dose of oseltamivir. The second was a double-blind, placebo-controlled, parallel-group, multiple-dose study in which participants were randomized to 100 mg or 200 mg oseltamivir or placebo (normal saline) i.v. over 2 h every 12 h for 5 days. Exposure to the active metabolite oseltamivir carboxylate (OC) after dosing achieved with 100 mg oseltamivir administered i.v. over 2 h was comparable to that achieved with 75 mg administered orally. Single i.v. doses of oseltamivir up to 400 mg were well tolerated with no new safety signals. Multiple-dose data confirmed good tolerability of 100 mg and 200 mg oseltamivir and showed efficacious OC exposures with 100 mg i.v. over 2 h twice daily for 5 days. These results support further exploration of i.v. oseltamivir as an influenza treatment option for patients unable to take oral medication.