MOSCOW, April 21 — At their first meeting with journalists since taking over Russia’s largest independent radio news network, the managers had startling news of their own: from now on, they said, at least 50 percent of the reports about Russia must be “positive.”

In addition, opposition leaders could not be mentioned on the air and the United States was to be portrayed as an enemy, journalists employed by the network, Russian News Service, say they were told by the new managers, who are allies of the Kremlin.

How would they know what constituted positive news?

“When we talk of death, violence or poverty, for example, this is not positive,” said one editor at the station who did not want to be identified for fear of retribution. “If the stock market is up, that is positive. The weather can also be positive.”

In a darkening media landscape, radio news had been a rare bright spot. Now, the implementation of the “50 percent positive” rule at the Russian News Service leaves an increasingly small number of news outlets that are not managed by the Kremlin, directly or through the state national gas company, Gazprom, a major owner of media assets.