The Russian space agency has blasted 10 gerbils into space for a 12-day mission to test the possible effects on humans of a flight to Mars.
Gerbils are more suited than most rodents for space flight
The small mammals, similar to mice and rats, are being kept in special cages with a supply of nuts and cereals.
Day and night will be simulated and special machines will clean their excrement in the weightless conditions.
The gerbils may find space preferable to returning to Earth - several are to be dissected upon their return.
The furry rodents lifted off from the Russian-run Baikonur space centre in Kazakhstan on Friday in a Soyuz rocket.
Gerbils are probably more suited than most rodents for space flight because they conserve body fluids by producing a minimum of waste.
The 10 are all sand rodents, praised as "a very interesting object for research" because they "can live for more than a month without using liquids," said Anatoly Grogoryev of the Russian Academy of Science.
"This will enable scientists to determine salt exchange mechanisms in zero gravity conditions."
Gerbils, as many pet owners have discovered, are gregarious and active in daylight, which makes them easy to observe.
The 10 gerbils will be filmed during their space flight.