UnitedHealth Group, Minneapolis, agreed to acquire PacifiCare Health Systems, Cypress, Calif., for about $8.1 billion in cash and stock in what would be the second-largest health-plan merger ever. The acquisition would give UnitedHealth a stronger presence in the West and a better foothold in the expanding Medicare market. PacifiCare is already one of the nation's largest Medicare managed-care plans and intends to become a key player in the administration of the new Medicare prescription-drug benefit. The acquisition -- involving about $2.2 billion in cash and 111.6 million shares -- is expected to close late this year or early next, pending approval by regulators and PacifiCare shareholders. UnitedHealth covers about 22 million members nationwide and earned $2.6 billion on revenue of $37.2 billion last year. PacifiCare has 3 million members, predominately on the West Coast, and earned $303.2 million on revenue of $12.3 billion last year.