British Olympic showjumper Michael Whitaker has withdrawn from Wednesday's third individual round after his horse Cassionato showed colic symptoms.
The horse, an 11-year-old grey stallion that has been ridden with considerable success by Whitaker, is being treated in a veterinary clinic and will take no further part at the event in Deodoro.
Colic in horses is a disorder of the digestive system - an abdominal pain - that in very serious cases can require surgery or even be fatal.
Great Britain showjumping performance manager Di Lampard said: "Obviously we're all very, very disappointed for Michael and owners Gary and Beverley Widdowson.
"Cassionato is being treated in the veterinary clinic this evening and is in very good hands. It just shows the effort the horse put into today's round."
Whitaker and Cassionato collected five faults during Tuesday's team round - four penalties at the water jump and a time penalty - as London 2012 team gold medallists Great Britain crashed out in round one.
The combination qualified for the Games' next individual phase in 42nd place and was due to compete again on Wednesday morning, local time.
Whitaker's team-mates Nick Skelton and Ben Maher both went through to the next individual phase, but now need strong performances to book a place in Friday's final day of equestrian action in Rio when individual medals will be decided.
The team medals, meanwhile, will be fought out by the United States, Holland, Germany, France, Brazil, Canada, Sweden and Switzerland.