Russian opposition activist Vladimir Kara-Murza, who faces up to 25 years in jail on charges including treason, was too unwell to attend his hearing on Thursday, his lawyer said.
His high-profile trial is the latest in a string of cases against opposition voices in Russia in a crackdown that has intensified since President Vladimir Putin sent troops to Ukraine last year.
"Today's hearing of the Moscow city court did not take place because Vladimir Kara-Murza could not be brought to court from the pre-trial detention center due to a significant deterioration of his health," lawyer Vadim Prokhorov said on Facebook.
He added that Kara-Murza has a nerve condition called polyneuropathy that was a "consequence of two severe poisonings in 2015 and 2017."
Prokhorov said that "even the doctors of the Russian prison system are beginning to worry" and said he would be making a request for Kara-Murza to be examined and treated "in a civilian hospital."
The next court date is scheduled for 20 March, but it is unclear whether Kara-Murza will attend, Prokhorov added.
Kara-Murza, 41, is accused of high treason, spreading "false" information about the Russian army, and being affiliated with an "undesirable organization."
He faces up to 25 years in jail, according to his lawyers.
The Western-educated activist and journalist was a close associate of opposition leader Boris Nemtsov, who was shot dead near the Kremlin in 2015, and Mikhail Khodorkovsky, a former oligarch turned Putin critic.
Kara-Murza says he was poisoned twice — in 2015 and 2017 — because of his political activities, but he continued to spend long periods in Russia.
Most Russian opposition figures are now either in jail or in exile.