A Russian capsule filled with 45 mice and 15 newts along with other small animals returned from a month's mission in orbit with data scientists hope will pave the way for a manned flight to Mars.
Russian Mission Control said the Bion-M craft landed softly with the help of a special parachute system in the Orenburg Region about 1,200 kilometres southeast of Moscow.
The capsule was also carrying snails and gerbils as well as some plants and microflora. There was no immediate information about how many of the animals survived.
The TsSKB-Progress space research centre's department head Valery Abrashkin said on the day the mission took off in April that the study was aimed at determining how bodies adapt to weightlessness "so that our organisms survive extended flights."
A field research lab has been deployed near where the capsule landed to quickly test the animals' response to their journey and return to Earth.
Scientists said the animals were needed because they were subject to the kinds of experiments that are impossible to be conducted on humans who are currently operating the International Space Station (ISS).
They added that the small menagerie would have posed a health risk if simply placed on board the ISS for a month.
The experiment's designers said the tests primarily focused on how microgravity impacts the skeletal and nervous systems as well organisms' muscles and hearts.
The capsule spun 575 kilometres above Earth.
Russia has long set its sights on Mars and is now targeting 2030 as the year in which it could begin creating a base on the Moon for flights to the Red Planet.
But recent problems with its once-vaunted space programme - including the embarrassing failure of a research satellite that Moscow tried sending up to one of Mars's moons last year - have threatened Russia's future exploration efforts.