Champion New Zealand stallion O'Reilly has died at Waikato Stud at the age of 21.
Stud proprietor Mark Chittick said O'Reilly died peacefully in his paddock on Wednesday morning.
"All good things must come to an end," Chittick said on the stud's website.
"O'Reilly has been wholly and solely responsible for the go-forward and momentum of Waikato Stud as we know it today.
"We could not be more proud of his achievements or more thankful for his legacy."
By Last Tycoon out of 1989 Golden Slipper Stakes Courtza, O'Reilly was the 1997 NZ Horse of the Year for his wins in the Group One Telegraph Handicap and Bayer Classic.
He had just completed his 17th breeding season.
Among his Group One winning progeny are Sacred Falls, Silent Achiever, Shamrocker, Shamexpress and Master O'Reilly.
Dual Doncaster Mile winner Sacred Falls will begin stud duties at Waikato Stud in the 2015 spring.