A person was gored and four others hurt, the Red Cross said, as Pamplona's bull running festival got under way.
One person was caught on a bull's horns during Saturday's run - statistically the most dangerous day of the week-long festival.
Four others were taken to hospital suffering minor injuries from falls.
The 875-metre run in the northern Spanish city lasts on average about two and a half minutes, with some 2,000 people taking part every year.
According to the Pamplona government, on average hundreds of people are injured every year during the festival, although few require medical treatment - falling and grazes are far more common than injuries caused by bulls or pile ups in the crowd.
Since records began in 1910, at least 16 people have died during the festival. The deadliest runs occurred in 1947 and 1980, when two participants in each year died.
The last person to die at the festival, Daniel Jimeno Romero, was gored in the neck in 2009 during the fourth run of the festival.
The majority of runners are men - women have only been allowed to take part since 1974.
This year's festival takes place amid protests about the treatment of women during the event.
A court case over sexual abuse at the festival rocked Spanish society this year, leading to protests nationwide.