Seitqazy Mataev, the head of the Kazakh Journalists Union and chairman of the National Press Club, has been admitted to hospital.
Mataev, who was charged with embezzlement and tax evasion, has been under house arrest since February.
Health officials at the Cardiology Center in Almaty told RFE/RL that Mataev was admitted with dangerously high blood pressure on April 7.
Mataev was detained by police on February 22. A court in Almaty ordered him to stay under house arrest until his trial.
Kazakhstan's Anticorruption Agency said that Mataev could face up to 12 years in jail if convicted on charges of embezzlement and tax evasion.
Mataev has said he is innocent.
Rights organizations have said Mataev was being prosecuted on trumped-up charges in response to his work as head of the journalists' union.