The move toward paid content comes at a time when online ad growth has spurred other publications--including CNN.com and LATimes.com--to offer more material at no cost, in hopes of offering advertisers more eyeballs.

Subscribers to the new premium service, dubbed "TimesSelect," also will have access to the archives, although the precise details have yet to be worked out, said Martin Nisenholtz, senior vice president, digital operations for The New York Times Co. Consumers who subscribe to the print edition of the Times will receive TimesSelect at no extra cost.

Currently, editorial content on NYTimes.com is free for the week following publication, while archived articles cost in the range of $1 to $3 each. The New York Times Co. has no plans at present to start charging for access to its other properties, including Boston.com and About.com, said Nisenholtz.

David Card, an analyst with Jupiter Research, called the decision to implement a subscription model "somewhat puzzling," given both a surging online ad market and the NYTimes.com's long refusal to charge for content. Even when the online ad market was in a slump, and other businesses migrated to a subscription model, the Times stuck with free offerings--which, Card said, makes Monday's announcement particularly ironic.

Nisenholtz indicated that the Times made the move away from free content to hedge its bets against setbacks in the online advertising market. "While advertising is growing very quickly, and we're all loving the growth, ultimately that advertising model is going to mature. It may be three years, it may be five years--but at that time, you'll want to have a second large revenue stream in the mix," Nisenholtz said.

But while subscriptions can generate additional cash flow, Card said that online advertising offers publishers greater potential revenue. "The advertising opportunity is the larger one, and always will be. They've been wise to exploit that up to this date," Card said. "It flies in the face of ... advertising recovery [to] take something that has previously had heavy traffic, and take it away," he added. The columns that will now be subscription-only frequently are listed as among the most e-mailed articles on the site. Nisenholtz said that the company will charge for those columns precisely because they are well-read. "Obviously, we're not going to try to sell something that's not popular," he said.