Technicians at the Kennedy Space Center in Florida began pumping 500,000 gallons (1.9 million liters) of liquid oxygen and liquid hydrogen into Atlantis' fuel tank in preparation for a launch attempt at 2:20 p.m. (1820 GMT).
Meteorologists predicted a 70 percent chance the weather would be suitable for liftoff.
Atlantis carries a small laboratory and docking compartment that will be attached to the Russian side of the space station, a $100 billion project of 16 nations nearing completion after more than a decade of construction 220 miles (354 km) above the Earth.
The Mini Research Module, nicknamed "Rassvet" - Russian for dawn - shares Atlantis' payload bay with a cargo carrier loaded with batteries, a spare communications antenna and an attachment for the station's Canadian-built robotic crane.
NASA intends to stock the station with as many spare parts as possible before turning over cargo resupply to smaller and less capable ships run by Russia, Europe and Japan.
NASA hopes commercial US carriers will eventually pick up the bulk of its station cargo business and possibly also crew transport, which is now provided solely by Russia.
To help get the station ready for operations after the shuttles are retired, the Atlantis astronauts plan to swap out six 365-pound (166 kg) solar array batteries on the station's main power truss, a task that will take the better part of two spacewalks.
Another spacewalk is to be devoted to installing a second communications antenna to the station's truss.
After Atlantis returns, NASA plans just two more trips to the space station with its shuttles.
Discovery is targeted for launch in September with spare parts and equipment. Endeavour is due to launch in November carrying a $2 billion, multinational particle detector known as the Alpha Magnetic Spectrometer.
Atlantis, which has flown 31 missions since it was put into service in 1985, will have one final assignment: remain on standby in case the last shuttle crew needs a rescue.
"It's a shame to be seeing it going away, but after 30 years it's probably time to move on," said Atlantis astronaut Stephen Bowen.
What will follow the shuttles is not yet known. Former President George W. Bush proposed a return to the moon under a program known as Constellation but shorted its funding. Congress approved the plan, but did not allocate enough money for the $110 billion project.
An advisory panel convened by President Barack Obama determined the program was unsustainable without an additional $3 billion per year into NASA's $18 billion annual budget.
Obama wants to end Constellation, boost NASA's budget by $1 billion a year and use the money to keep the space station flying beyond 2016, when it had been scheduled for retirement.
He has proposed investing in new propulsion technologies and heavy-lifting rockets, with the interim goal of visiting an asteroid and a long-term goal of landing astronauts on Mars.
In addition to Bowen, the Atlantis crew includes commander Kenneth Ham; pilot Dominic "Tony" Antonelli; flight engineer Michael Good; and mission specialists Garrett Reisman and Piers Sellers. All are making their second spaceflights, expect Sellers, a British-born astronaut flying for the third time.
