RSS chief KS Sudarshan was on Thursday admitted to a hospital in Nagpur with viral fever and underwent medical tests.
Dr Harshvardan Maldikar of Spandan Heart Research Institute, where 76-year-old Sudarshan was admitted at around 1730 hours, said the RSS chief was down with viral fever.
He said Sudarshan, who had been suffering from fever for the last two days, was advised hospitalisation since the temperature was not coming down.
A statement issued by RSS said Sudarshan was having temperature ranging from 100 to 101 for the last two days and since it started rising again on Thursday despite some medication, he was taken to the hospital.
Maldikar said Sudarshan would be subjected to tests and check-up again Friday.
In Delhi, RSS leader Ram Madhav said "there's nothing serious. It's just fever and Sudarshan is likely to be discharged tomorrow after medical examination."
Former Prime Minister Atal Bihari Vajpayee and senior BJP leader LK Advani enquired about Sudarshan's health.