An American who co-wrote a book called Fiesta: How To Survive The Bulls Of Pamplona has become one of their victims after he was gored at the festival.
Bill Hillmann, a 32-year-old from Chicago and a longtime participant in the nine-day Pamplona street party, was gored twice in the right thigh during one of the daily bull runs, organisers said on their website.
The injury was serious but not life-threatening, the Navarra regional government said.
A 35-year-old Spaniard from Valencia was also in serious condition after being gored in the chest during the same run on the festival's third day, the statement said.
Photographs showed Mr Hillmann - dressed in the San Fermin event's traditional white with a red neckerchief - being gored on the ground by a black bull as other runners scattered.
Tension spiked when the bull became separated from the pack in the final stretch.
British writer Alexander Fiske-Harrison, one of Mr Hillmann's co-authors and a fellow runner, said on his blog that the bull turned back and charged at runners.
Mr Hillmann's wife Enid was at the hospital with her husband, Mr Fiske-Harrison said.
Yesterday, authorities said a 23-year-old British man from Nottingham was in serious condition with chest injuries and rib fractures
The man - identified only by his initials, TH - was hurt in the second running, along with a Japanese man and a Spaniard. One person was gored in the opening run.
Dozens of people are injured each year in the runs, most of them in falls. Fifteen people have died from gorings since record-keeping began in 1924.
The bulls are invariably killed in afternoon bullfights in the ring.
The nine-day street-partying festival was immortalised in Ernest Hemingway's 1926 novel The Sun Also Rises and attracts thousands of foreign tourists.