Aetna shares soared by a much as 12 percent Thursday when news of the deal first broke.
CVS Health has proposed to pay more than $200 per share for Aetna, the sources familiar confirmed to CNBC.
Aetna, which as of Thursday had a market value of $53 billion, was trading at about $175 per share after news.
CVS Health shares were down nearly 3 percent for the day and continued lower in premarket trading Friday.
In addition to a chain of retail pharmacies, CVS Health also operates the Caremark pharmacy benefit manager, which has more than 75 million plan members.
Asked about the potential deal, an Aetna spokesman told CNBC, "We don't comment on rumor or speculation."
A spokesman for CVS Health said, "As a matter of policy we don't comment on market rumors such as this."
The Wall Street Journal first reported the proposed acquisition.
Aetna in February abandoned an effort to merge with another large health insurer, Humana, after that $34 billion deal was blocked on antitrust grounds by a federal judge.