Moody's said that it anticipates that any downgrade would likely be limited to one notch, based on the financing terms disclosed.
The ratings agency said that Ralcorp's ratings are under review for a possible upgrade. That includes the St. Louis company's "Baa3" long-term senior unsecured rating.
Moody's said the potential upgrade is because the acquisition may result in a combined company that has a stronger credit profile than Ralcorp currently has.
Moody's also said Ralcorp's debt ratings may be upgraded one notch.
ConAgra shares rose $1.34, or 4.7 percent, to close at $29.63. Ralcorp surged by $18.57, or 26.4 percent, to close at $88.80.