Scientists are conducting experiments to change the sexuality of "gay" sheep in a programme that critics fear could pave the way for breeding out homosexuality in humans.
The technique being developed by American researchers adjusts the hormonal balance in the brains of homosexual rams so that they are more inclined to mate with ewes.
It raises the prospect that pregnant women could one day be offered a treatment to reduce or eliminate the chance that their offspring will be homosexual. Experts say that, in theory, the "straightening" procedure on humans could be as simple as a hormone supplement for mothers-to-be, worn on the skin like an anti-smoking nicotine patch.
The research, at Oregon State University in the city of Corvallis and at the Oregon Health and Science University in Portland, has caused an outcry. Martina Navratilova, the lesbian tennis player who won Wimbledon nine times, and scientists and gay rights campaigners in Britain have called for the project to be abandoned.
Navratilova defended the "right" of sheep to be gay. She said: "How can it be that in the year 2006 a major university would host such homophobic and cruel experiments?" She said gay men and lesbians would be "deeply offended" by the social implications of the tests.