The deal comes just a day after another US media giant NBC Universal paid $600m for women's portal iVillage.
Hearst, which owned a 25% stake in iVillage, had been named as one of the potential bidders in that race along with publishing rival Hachette Filipacchi.
The acquisition of NetDoctor, which will become part of Hearst's UK operation, the National Magazine Company, sees the Cosmopolitan, Harpers and Esquire publisher joining the ranks of media firms snapping up online businesses.
Duncan Edwards, chief executive of NatMags, said: "We have been watching the digital publishing sector very closely over the last few years and have now decided it is time to ramp up our activity."
Edwards said that the purchase of NetDoctor comes at the start of a busy period of digital activity, where it will launch more sites on the web and look for further acquisitions.
"We are interested in building a business in digital publishing and have major plans to develop that side of our operation in coming months.
"Our first new-look magazine website, youandyourwedding.co.uk was launched last week with a full suite of content and services for the bride and groom and gives a good sense of the quality of what will be produced for all our titles over the next year," he said.
He added that community would be a major focus for NatMags mirroring the interest showed by other parts of the industry in acquiring community based websites such as iVillage and News Corporation's acquisition of Myspace.com.
NetDoctor, which launched in February 2000, allows visitors to find out more about diseases, take health tests, look up medicines and check side effects. One facility lets users ask experts for an opinion on any ailment.
The site is already reported to be a profitable business and attracts around 2m unique visitors a month. In the past, it has worked with the likes of Associated Newspaper's Femail.co.uk.
According to Edwards, NatMags will promote NetDoctor, which will operate as a standalone business, in its magazines and on its websites to drive more traffic.
"It's like the gold rush again. We intend to throw our full weight behind the development of a scale digital publishing business and will use all the marketing power of our magazine properties in building the websites for our own magazines and for NetDoctor," Edwards said.
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