Aetna Inc. (NYSE:AET) will pay about $5.7 billion in cash and stock to buy Coventry Health Care Inc. (NYSE:CVH), a managed health care company with a big market share in Medicare- and Medicaid-related products.

Hartford, Conn.-based Aetna will borrow $2.5 billion to complete the transaction, which will add nearly 4 million medical members and 1.5 million Medicare Part D members to Aetna's membership.  On a pro forma basis, the transaction increases Aetna's share of revenues from U.S. government business to more than 30 percent from 23 percent currently.

Shares of Coventry, which is based in Bethesda, Md., jumped $6.51, or 18.63 percent, to $41.45 in premarket trading, well above the stock's 52-week range of $25.78 to $36.04. Shares of Aetna rose 76 cents, or 2 percent, to $38.80.