KKR will pay $66.50 per share, a premium of 20.5 percent to WebMD's Friday closing price. WebMD's shares were trading at $66 before the opening bell.
Founded in 1996, WebMD has grown into one of the most popular health websites for consumers and medical professionals, attracting more than 70 million monthly unique visitors in 2016, according to analytics company comScore Inc.
WebMD's Medscape, a medical news and education website, accounted for about 60 percent of the company's advertising revenue in 2016.
Internet Brands, which launched as CarsDirect.com in 1998, licenses and delivers its content and internet technology products and services to small and medium-sized businesses.
It was acquired by KKR in 2014 for $1.1 billion from two other private equity firms, Hellman & Friedman LLC and JMI Equity.
Reuters reported on Sunday that KKR was nearing a deal to buy the online health information provider.
The deal, approved by the WebMD board, is expected to close in the fourth quarter of 2017.
WebMD also announced preliminary second-quarter sales that were higher than the average analyst estimate.
J.P. Morgan Securities LLC was WebMD's financial adviser, while Shearman & Sterling LLP was its legal adviser. Simpson Thacher & Bartlett LLP was Internet Brands' legal adviser.
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