The underground visitor centre at the 13th century Urquhart Castle, near Loch Ness, will be monitored for three months.
It follows concerns at the nearby Glen Urquhart High School, Drumnadrochit, where levels of the naturally- occurring gas were measured at seven times above national guidelines.
Long-term exposure to radon is widely acknowledged as the most frequent cause of lung cancer after smoking.
The medieval fortress is Historic Scotland's third most popular attraction with more than 300,000 visitors a year.
Sensors will be installed inside the underground visitor centre this week to gather data for three months.