With the offering, the large U.S. pharmacy benefit manager intends to swap out the stock portion of the $4.68 billion acquisition in favor of cash.
Express Scripts estimated proceeds from the offering would be $1.4 billion, or $1.6 billion if underwriters fully exercise their over-allotment option. The 23 million shares represent more than 9 percent of the company's shares outstanding as of the end of March.
Express Scripts agreed in April to buy WellPoint's NextRx business to become the No. 2 U.S. pharmacy benefit manager. The deal is expected to close late in the third quarter or in the fourth quarter.
Express Scripts said it also intends to offer senior notes "of a benchmark size" in the near term to help pay for the deal.
Express Scripts shares closed at $64.45 on Monday on the Nasdaq. The shares have risen 17 percent this year.