Low-lying clouds over the Kennedy Space Center, which moved in Saturday night, were pinpointed as the factor.

"I'm not comfortable for launch tonight," Leinbach said.

The launch has been rescheduled for 3:14 a.m. (CST) Monday.

With the exception of the weather problems, which would have impacted the Endeavour had an emergency return to the Cape been required, all systems were working perfectly during the countdown.

There are only four Space Shuttle flights scheduled after the Endeavour. This is the last night-time launch planned.