A prominent critic of the Kremlin was rushed to hospital with organ failure, two years after he nearly died from suspected poisoning.
Vladimir Kara-Murza, who works for pro-democracy group Open Russia, was taken to an intensive care unit in Moscow.
The activist and journalist had spent the previous night at his in-laws home.
Mr Kara-Murza's symptoms are "similar to those he had two years ago,” his wife Evgenia told Radio Free Europe (RFE).
"His doctors describe his condition as critical," she said. "He has low blood pressure and respiratory insufficiency, and the reason for this is yet again unclear."
In a separate interview with the BBC, she said he was on life support and in “a medicated coma”.
The 34-year-old has been travelling around Russia in recent weeks to promote a documentary about his friend Boris Nemtsov, a liberal opposition politician who was assassinated near the Kremlin in 2015.
The day before falling ill, Mr Kara-Murza paid tribute to Mr Nemtsov on Facebook.
No cause was ever established for Mr Kara-Murza’s illness in May 2015, though tests confirmed he had ingested a poisonous substance.
He was in a critical condition for several days and spent two months in hospitals in Moscow and the US as he recovered.
Mr Kara-Murza has “no doubt” his ordeal two years ago was a “deliberate poisoning aimed to kill”, and that it was politically motivated.
"I was a healthy 33-year-old man. Suddenly, when all of your major organs shut down within 24 hours, one after another -- that doesn't just happen,” he told RFE.Reuse content