It is the biggest event on the bullfighting calendar, a week of non-stop parties, processions and bull runs that can make or break a matador's reputation.

This year, though, the San Isidro festival, in Madrid, will feature a distinctly modern twist: bulls will be subjected to strict doping tests to reveal if they have been given banned drugs.

Laboratory technicians will not, however, be looking for the type of performance-enhancing drugs taken by athletes. Officials fear that bulls are being drugged with tranquillizers to make them less aggressive, giving the matador a crucial edge in the contest.

The week-long festival begins on May 8. It will be the first time that doping tests have been carried out at Las Ventas bullring, regarded by aficionados as a spiritual home of the sport. According to El Mundo newspaper, only bulls that are exhibiting “strange behavior” in the ring will be tested. Blood and urine samples will be analyzed in Madrid and, if they test positive for a banned substance, will be checked against a “B” sample. Any breeders found to be drugging their bulls face a hefty fine and the loss of their reputation in the fiercely proud world of bullfighting.

The image of a strutting bullfighter engaged in a dance with death, as immortalized by Ernest Hemingway, is synonymous with Spain. Many bullfighting fans complain, though, that bulls today are a shadow of the beasts of yesteryear and stand little chance against the matadors.

One of Spain's leading critics said that the bulls at Seville's fair earlier this month were “shameful,” deriding them as “kittens” and calling one fight “an afternoon best forgotten."

“At first sight, they looked like bulls,” wrote Antonio Lorca in El País newspaper, “but in fact they were kittens. Instead of roaring, they mewed. They aroused pity, rather than respect.”

There has been a long tradition of tampering with bulls in an effort to tip the balance in favor of the matador. Starting in the 1940s, allegations surfaced that breeders were “shaving” their bulls horns, a practice that disorients bulls and places them at a disadvantage. The practice, which WSPA International, the animal rights group, likens to “grinding someone's teeth away without anesthetic,” was outlawed by the Government in 1992.

In 1952 the matador Antonio Bienvenida denounced the practice in public and refused to fight any bull that had been tampered with. Many other bullfighters refused to talk to him and he became something of an outcast. In 1985 a vet at Las Ventas also went public with allegations of doping after seeing several of the bulls apparently losing their balance during the fights. Tests the following year revealed the presence of tranquillizers in the bloodstreams of several bulls.

Campaigners against bullfighting said yesterday that the need for tests revealed the unfairness of the contest between man and bull.

Alyx Dow, the anti-bullfighting programs officer at WSPA, said: “We are concerned that the organizers are using this new step to distract from the other cruel methods used to weaken the bull in the ring and the fact that all the bulls will ultimately experience prolonged suffering and a painful death.”