UPDATE 2-Aetna, UnitedHealth win defense contracts

* Aetna contract $2.8 bln; total potential $16.7 bln

* UnitedHealth $3.7 bln; total potential $21.8 bln

* Humana, Health Net are replaced

* Humana, Health Net shares fall; Aetna, UnitedHealth up (Adds share prices, paragraph's 2-3)

NEW YORK, July 13 (Reuters) - Aetna Inc (AET.N) and UnitedHealth Group Inc (UNH.N) won new multibillion dollar contracts on Monday to provide services under a U.S. defense department's health care plan, replacing rivals Health Net Inc (HNT.N) and Humana Inc (HUM.N).

Aetna replaced Health Net for the northern region of the the department's TRICARE program, while UnitedHealth will take over from Humana in the south. The new contracts are expected to begin on April 1, 2010.

Health Net's shares fell 15.1 percent, while Humana's were down 4.3 percent in aftermarket trading. Aetna's shares rose 2.3 percent and UnitedHealth was up 2.4 percent in aftermarket trading.

Aetna was awarded a $2.8 billion managed care contract with five one-year options for a total potential value of $16.7 billion, according to the department's website.

The company could generate gross revenue of about $1.1 billion over the five-year agreement, an Aetna spokesman said.

UnitedHealth got a $3.7 billion managed care contract with five one-year options for a total potential value of $21.8 billion. The contract is for the southern region.

Not-for-profit TriWest Healthcare Alliance retained its contract for the western region. It got a $2.9 billion contract with five one-year options for total potential value of $17 billion.

In a separate statement, Health Net said it was disappointed not to be selected but that it would decide whether or not to challenge the decision after a debriefing.

TRICARE administers the health care plan for 9.4 million members of the military, retirees and their families worldwide. The contractors provide health, medical and administrative support services. (Reporting by Paritosh Bansal, Editing by Phil Berlowitz, Leslie Gevirtz)