WASHINGTON — Shares of AT&T Inc. and Time Warner Inc. fell Monday as investors didn’t appear thrilled by the proposed mega-merger announced over the weekend.

In response to the $84.5 billion deal, Moody’s Investor Service said Monday it had placed AT&T’s credit rating on review for a possible downgrade.

As speculation about the deal swirled Friday, AT&T shares slid 3 percent and Time Warner shares leaped 7.8 percent. In trading Monday, the stocks closed down 1.7 percent and 3.1 percent, compared with their Friday closing prices.

Moody’s said the deal is expected to face “a rigorous regulatory review” in Washington and that approval could come with “conditions that could limit AT&T’s ability to use Time Warner’s content as a competitive advantage.”

“Regulatory conditions could ultimately undermine AT&T’s objective to differentiate its mobile and pay TV platforms with exclusive content,” the credit rating company said.

Moody’s said it expected any downgrade to be just one notch down from the current Baa1 rating on AT&T’s senior unsecured debt. The current rating is the eighth-highest given by Moody’s and is just below the A level.