The Japan Racing Association says it has scratched 156 horses from races this weekend after discovering that the animals may have been fed a banned substance.
Officials say they pulled the horses from races on Saturday and Sunday at tracks in Tokyo, Hanshin in western Japan, and Hakodate in northern Japan.
The association says it found that dietary supplements that were sold to its stables contained theobromine, a banned substance. The supplements were to be mixed with food.
Theobromine can act as a stimulant and is thought to give horses an advantage in racing.
The association says it can't rule out the possibility that the 156 horses may have ingested the substance.
About 1,000 horses were scheduled to take part in 72 races at the three racecourses over the weekend.
The association says that on average, each race will have two fewer horses.
It says it is unprecedented for so many horses to be scratched from races in connection with a banned substance.