Investigators told Russian news agencies that they were conducting chemical tests to determine whether 43-year-old Anatoly Popov had been the victim of an attempted poisoning.
Popov was rushed to a local hospital in Chechnya early on Sunday morning. He fell ill after a meal in the eastern town of Gudermes, just a week before Kremlin-sponsored presidential elections in the restive province.
Popov was later flown to Moscow, where he told the Interfax news agency that his condition was "stable" and that he planned to return to Chechnya the following day.
Popov is deputy to Ahmad Kadyrov, who is backed by the Kremlin and widely expected to be elected as the region's president.
A Russian soldier stands guard in
front of a military helicopter
All other major candidates have been either barred from running or otherwise removed.
Meanwhile, violence continued to rock the war-ravaged region.
On Monday two Russian soldiers died and one was injured after a mine exploded under the vehicle in which they were travelling in the southeastern Vedeno area, Interfax reported.
A policeman was also killed and three others wounded in a shootout at the village of Aiti-Mokhk in the Nozhai-Yurt district after a group of more than 10 men attacked a police unit deployed at a polling station, it added.
Chechen separatists have vowed to increase their attacks on Russian targets ahead of the election, which will take place on 5 October.