Men in the UK are being recruited to try out a new form of male contraception in a major study.
Men have to have testosterone injections every three months
The hormonal treatment uses a combination of an implant under the skin and regular injections to lower sperm production temporarily.
The European trial, involving centres in London and Manchester, is one of several hoping to develop a 'male Pill'.
But it will be some years before any are widely available.
Men who want to take responsibility for contraception currently have a choice of condoms, vasectomies - or abstinence.
Researchers involved in this latest study believe the development of a male contraceptive treatment would offer a "new concept" in family planning.
They say earlier tests suggest the implant could be as reliable as the female contraceptive pill, and that it does not cause side effects.
The treatment, developed by the pharmaceutical companies Schering and Organon Laboratories, works by implanting a tube under the skin that slowly releases the female hormone progestogen.
Every three months, men are also given intra-muscular injections of the male hormone testosterone.
The combination of the two hormones temporarily turns off the normal signals from the brain that stimulate sperm production.
But it also switches off the man's own testosterone production - so he needs to be given extra doses of the hormone to keep him healthy and maintain his sex drive.
Once a man stops taking the treatment, hormone levels - and therefore sperm production - should return to normal.
In the forthcoming trial, men will be given different hormone combinations to see which is most effective.
Fourteen centres across Europe will recruit around 350 men for the research.
Once the most effective hormone combination has been found, researchers will have to carry out further, larger scale trials before the treatment can be licensed.
Dr Pierre-Marc Bouloux, who is leading the research at the Royal Free Hospital in London, told BBC News Online: "We are looking at different combinations of hormones to see if we can reduce sperm production to a level that's guaranteed, and achieve the goal of reversible infertility."
He added: "Instead of defining success at a lack of conception, which can be affected by many factors, we'll be looking at sperm production levels."
A spokesman from the Family Planning Association said: "This is an important development in the contraceptive choices that are available to couples and we look forward to seeing the results of the trials across the country.
"Many men want to play more of an active role in contraception and FPA is pleased that we are moving towards this becoming more of a possibility."
Men who want to take part in the study can call 0207 472 6190.