Indianapolis-based WellPoint is the nation's second-largest health insurer, covering 35 million individuals.
WellPoint's pharmacy subsidiaries, called NextRx, dispense and manage 265 million prescriptions each year for 25 million people.
As part of the sale, Express Scripts has agreed to a 10-year contract to provide services to WellPoint.
"Importantly, through this strategic alliance with Express Scripts, we will enhance the health care value we bring to our members," said WellPoint CEO Angela Braly. "This alliance will create an organization with greater resources and capabilities, which will provide members with more cost-effective solutions as well as access to state-of-the-art [pharmacy management] services."
WellPoint will retain control of medical policy, formulary and integrated disease management aspects of its pharmacy benefits.
The deal is expected to close in the second half of 2009. Express Scripts will pay WellPoint $3.275 billion in cash and $1.4 billion in stock. The deal also includes compensation for the value of a future tax benefit for Express Scripts based on the structure of the deal.