"There can be no assurance that an agreement will be reached or a transaction consummated," Healtheon said in a statement.
Shares of Healtheon edged 1.79 percent higher in early trading to 56.88. The stock has traded as high as 63.5 and as low as 21.75 during the past 52 weeks.
Speculation about merger talks has been spreading for several weeks now. Analysts have previously noted that the companies are strategically complementary.
Healtheon focuses on facilitating health care transactions over the Internet and via other proprietary services while WebMD offers medical information and services to physicians and consumers.
The companies hope a merger will help form a dominant provider of Internet services that link patients, suppliers, and insurers and the merger also would form a company that is owned equally by Healtheon and WebMD, a person familiar with the transaction told Bloomberg.
Subsequently, Microsoft said that it had extended until May 25 an offer to purchase shares of WebMD's Series F Preferred Stock, and is sending notice to stockholders. The software giant "expects to make a further announcement regarding its offer in the event Healtheon and WebMD reach agreement on their proposed merger," the company said in a statement.
As of today, holders of 499,790 shares of Series F Preferred Stock had accepted Microsoft's purchase offer, the company said.