An Ameren spokesman says the inspection is unrelated to heightened concerns at nuclear plants following the damage to the plant in Japan.
The NRC says an oil sample taken Feb. 8 showed the auxiliary pump might have been inadequately lubricated.
The pump is used to supply water to steam generators during some accident conditions.
The oil sample indicated that the oil level may have been too low to properly lubricate the pump bearing. If that happens, the pump may not be able to run long enough during an accident scenario.
The NRC says the inspection was begun because a similar finding occurred at Callaway in 2009.