Viktor and Marina Kalashnikov are freelance journalists and commentators who have strongly criticized the Kremlin. The German magazine "Focus" reported that doctors had found dangerous levels of mercury in their blood after they left Russia in September.
The magazine said they had suffered serious headaches and pains and that Marina had lost half of her hair. Tests showed both had more than 50 micrograms of mercury per liter in their blood, far above the normal level of three micrograms.
Kalashnikov has blamed the Kremlin. "Moscow poisoned us," he told "Focus."
German investigators say their probe is being carried out by a department that specializes in politically motivated crimes.
News of the suspected poisoning is reviving memories of the radioactive poisoning of former KGB officer Aleksandr Litvinenko. The prominent Kremlin critic died in London in 2006.