An American who co-wrote the book How to Survive the Running of the Bulls was badly gored in the morning run at Pamplona's Festival of St Fermin yesterday.
A Spanish man was also gored in the encierro, during which runners dressed in white with red scarves are pursued through the streets by bulls.
Chicago resident Bill Hillmann, 32, was gored in the right thigh, festival website sanfermin.com reported.
The Spaniard, a 35-year-old from Valencia, was gored in the chest and taken to hospital semi-conscious, officials said.
Both men are in a serious condition.
Another three Spaniards were taken to hospital with lesser injuries inflicted during the chaotic stampede through the narrow streets of Pamplona's old town.
All the injured were men, reports said. Few women take part in the run.
Yesterday's run was the third in the week-long festival, depicted in Ernest Hemingway's 1926 novel The Sun Also Rises.
Hemingway aficionado Hillmann travels to Pamplona every year to take part in the festival.
The black bull that gored him was the heaviest of the morning's six from the Victoriano del Rio ranch, weighing about 600kg.
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