A new contraceptive pill for men has been developed promising to block fertility protein for 24 hours.
Scientists said the new pill is more effective than women's oral birth control medications which have to be taken daily.
The drug temporarily disables an enzyme called sAC (soluble adenylyl cyclase) which triggers cells to swim.
In experiments, male lab rodents mated with females - but there were no pregnancies.
The non-hormonal compound stopped mouse sperm cells in their tracks - preventing them from maturing.
Lead author Dr Melanie Balbach said: "Our inhibitor works within 30 minutes to an hour.
"Every other experimental hormonal or non-hormonal male contraceptive takes weeks to bring sperm count down or render them unable to fertilise eggs."
"Sperm recovered from female mice remained incapacitated. There were no side effects.
"The compound wore off three hours later, and males recovered their fertility."
A single dose rendered sperm immobile for up to two-and-a-half hours - with the effects persisting in the female reproductive tract after sex.
There were 52 attempts at impregnation and all had failed.
The team at Weill Cornell Medicine, New York, hailed the breakthrough as a potential "gamechanger".
For years scientists have worked to develop an effective male oral contraceptive for decades.
Previous contraceptives had led to obesity, depression and high cholesterol.
Women's choices range from pills to patches to intrauterine devices but they bear most of the burden of preventing pregnancy.
There are only two contraceptives for men currently - condoms or a vasectomy.
Condoms are only single-use and prone to failure. A vasectomy is a surgical sterilisation that's expensive to reverse and not always successful.
It takes weeks to reverse the effects of other hormonal and non-hormonal male contraceptives in development, said Dr Balbach.
It could also allow men to make day-to-day decisions about their fertility.
Co-author Professor Lonny Levin said: "The team is already working on making sAC inhibitors better suited for use in humans."
The team said it planned on repeating its study in a different pre-clinical model to lay the groundwork for clinical trials.
It would test the effect on sperm motility in healthy human males.
Prof Levins said he hopes to walk into a pharmacy one day and hear a man request "the male pill".
