Two Americans and a Spaniard were gored on Friday at the first running of the bulls at the annual San Fermin fiesta in Pamplona, organisers said.
"A Spaniard from the Navarro region... is in a serious condition and two Americans (are) in a less serious condition," the festival said in a statement.
The nine-day fiesta, which kicked off on Thursday, dates back to the Middle Ages, commemorating a local saint.
Crowd of revellers, dressed in white and red and often fuelled by copious amounts of wine, sprint down the narrow streets of Pamplona pursued by six bulls.
The three-minute, 850-metre (yard) ritual was immortalised by Ernest Hemingway in the 1926 novel "The Sun Also Rises."
Fifteen people have died since 1911, with the last fatality occurring in 2009. In 2016, 87 were injured, many of them gored or trampled by the bulls.