Reuters quotes an official in the Moscow Patriarchate as saying that Aleksy received treatment while on holiday.
The official did not specify what was wrong with the patriarch, but said he would resume his working schedule in May.
In comments to Interfax, Orthodox Church official Sergei Kravets dismissed suggestions that Aleksy's health may be deteriorating.
However, the Echo Moskvy radio station quoted a source in Switzerland as saying that the condition of the 78-year-old patriarch was "rather serious."
Aleksy II has had a history of heart problems.
(compiled from agency reports)