Thousands of runners will put themselves through 20 miles of mud and obstacles, many hoping to raise hundreds for charity.
The Rat Race Dirty Weekend takes place for the third time in the grounds of Burghley House on Saturday next week.
Those taking part can take on a course of either 13 or 20 miles featuring a huge range of obstacles, from muddy trenches to huge wooden climbing frames.
Competitors will come from across the country and beyond. Most will camp out in Burghley Park and, if their muscles still work, will head to the big top tent for music from indie band Ash and funk and soul DJ Craig Charles.
Many runners are taking part for charity. Brian Makwana, from Castle Bytham, and a team of friends are running to raise money for CDH UK. Brian’s one-year-old son Wyatt was born with a condition called congenital diaphragmatic hernia, or CDH, and spent most of his first year in hospital.
And Barry Newman, part of 16th Regiment Royal Artillery, based at St George’s Barracks in North Luffenham, is raising money for Children with Cancer UK, which is the event’s chosen charity.
Organisers also hope to break the Guiness World Record for the most people running in their underwear. The attempt, named Dirty Undies, will take place on Friday next week.
The record currently stands at 2,270, which was set in 2011 at the Utah Undie Run in the US.
Are you taking part? Send us your photos after the event and you could feature in the paper. E-mail the newsdesk.