It is the fourth year the event has been staged in Pamplona in protest against the treatment of bulls, who are stampeded through the streets, prior to the bullfights in which they will be killed.
This year more than 200 nude or nearly-nude activists, including six from West Yorkshire, are to take the same route as the bulls on July 5, a few days prior to the traditional "bull-running".
Although some runners opt to wear white bikinis, Sabina Arthurs, of Brighouse, said she planned to strip completely.
Miss Arthurs said: "I think it's important to make an impact. I'm not bothered about the cameras, we have to get the message across.
"I've been to quite a few demonstrations but this is the first one I've done nude.We want to show people that you can have fun and attract tourists with something that's lighthearted.
"Hopefully we will get the bull-running banned, but if not, I'll be going back next year."
Gurt Bains, a Huddersfield University student, is going back for her second year at the event, known as the "Human Race". She said: "The locals supported it last year. They were all cheering and clapping. It was fun."
The event is organised by PETA, People for the Ethical Treatment of Animals.
Director of European Campaigns Sean Gifford said: "The Running of the Bulls is tourist-driven, but most visitors haven't a clue about the cruelty that bulls are subjected to. Electric-shock prods and sharp sticks are used to frighten the bulls into a stampede. As the animals flee, they sustain bruises, cuts and broken bones. In the bullfights bulls are often intentionally debilitated with tranquilisers and beatings and have petroleum jelly rubbed in their eyes so they are less able to resist.
"Tourists flocking to Pamplona are looking for a thrill, and our Running of the Nudes aims to give them just that – without harming a hair on a bull's back."
amy.binns@ypn.co.uk