Standard & Poor's Ratings Services on Friday placed an "AA+" rating on $8 billion of notes issued by Berkshire Hathaway Inc.
The rating is the next to the highest long-term credit rating S&P offers.
The notes issued Thursday will be used to help Warren Buffett's investment company buy the remaining shares of Burlington Northern Sante Fe Corp. in a cash and stock deal valued at $26.3 billion. Berkshire Hathaway already owns 22 percent of the stock.
The acquisition of the nation's second-largest railroad would be the biggest ever for Omaha, Neb., based Berkshire.
Shareholders of BNSF are scheduled to vote on the proposed acquisition Feb. 11. The deal is expected to close by Feb. 15.
The notes will be sold in a series of three floating-rate and three-fixed rate issuances. Maturity dates range from 2011 to 2015.
Standard & Poor's lowered its long-term counterparty credit rating on the company to "AA+" from "AAA" on Thursday and removed the rating from CreditWatch, where it was placed with negative implications on Nov. 4, 2009.
S&P said it expects a significant part of the cash portion of the acquisitions costs to come from Berkshire Hathaway's core insurance operations, which will reduce their liquidity.
S&P said Berkshire's financial flexibility remains extremely strong.
The downgrade left just four U.S. industrial companies with S&P's "AAA" rating: Microsoft Corp., Exxon Mobil Corp., Johnson & Johnson and Automatic Data Processing Inc. More than a dozen U.S. financial institutions, including the Knights of Columbus and New York Life Insurance Co., hold the highest designation.
Berkshire's class B shares rose 96 cents, or 1.3 percent, closing at $73.57. They gained another 3 cents in aftermarket trading.