The course for the inaugural Salomon Glen Coe Skyline race features an ascent of the Munro Buachaille Etive Mor.
Organisers had warned in advance that the severe terrain involved posed a risk of "serious injury or death" to those entering the event.
The event began at 07:00 and will go on until 21:00.
Athletes from France, Belgium, Spain and across the UK entered their names for the Glen Coe Skyline.
It is one of six races in the 2015 Skyrunning UK calendar.
The other events include races in the Lake District in England and Mourne Mountains in Northern Ireland.
More than 170 athletes are taking part in an endurance event which pits them against the Scottish mountains.