The prospect of a "male pill" that would let men enjoy a full sex life with no chance of getting a woman pregnant has moved a step closer.
Scientists in Australia have found a reversible way to stop sperm getting into the ejaculate, without affecting sexual function.
The animal tests showed the sperm could be "kept in storage" during sex.
The findings were published in the journal Proceedings of the National Academy of Sciences.
The quest for the male contraceptive pill has largely focused on getting men to produce non-functional sperm.
But some drugs used for this purpose "have intolerable side-effects," said Dr Sabatino Ventura, one of the researchers at Monash University.
Drugs can induce infertility, but they may also affect sexual appetite or cause permanent alterations to sperm production.
The team at Monash used a different approach. Normally, the sperm is moved out of the vas deferens storage area in the testes just before ejaculation.
The group produced genetically modified mice that were unable to squeeze the sperm out of the vas deferens.
Dr Ventura told the BBC: "The sperm stay in the storage site so when the mice ejaculate there's no sperm and they are infertile.
"It is readily reversible and the sperm are unaffected, but we need to show we can do this pharmacologically, probably with two drugs."
So far the research group has made the mice infertile by changing their DNA to stop them producing two proteins needed to move the sperm.
The researchers now need to find a pair of drugs that can produce the same effect. They believe one has already been developed and has been used for decades in patients with benign prostate enlargement.
However, they would have to work from scratch to find the second one - a process that could take a decade.
The proteins targeted also have a role in controlling blood vessels; so there could be side-effects on blood pressure and heart rate.
However, in the mice at least, the researchers detected only a "very slight" drop in blood pressure. There could also be an impact on the volume of ejaculate.
Dr Allan Pacey, senior lecturer in andrology at the University of Sheffield , told the BBC: "It's a very good study, almost like a biological vasectomy in [that] it stops the sperm coming out.
"It's a good idea; they need to get on with it and see what it does in people."
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