The Grand National course at Aintree will be changed for 2012 after a review of
April's race in which two horses were fatally injured.
A reduction of between four and five inches to the drop on the landing of Becher's Brook is among the changes.
"It is not possible to completely eliminate risk in horse racing," said Aintree managing director Julian Thick.
"However, I am confident the course changes we are announcing will, over time, have a positive impact."
The fourth will be reduced in height by two inches to 4ft 10in (1.47 metres) after a statistical review found that it and Becher's were the most difficult jumps to clear on the course.
Both fences were omitted from the second circuit in this year's edition of jump racing's showpiece after Dooneys Gate and Ornais suffered fatal falls on the first lap.
DID YOU KNOW?
- Twenty horses have died on the Grand National course since 2000 according to Animal Aid
The drop on the landing side of the first will also be reduced while the height of the toe boards around the course will increase to make it easier for horses to determine the base of the fences.
"Raising the landing zone at Becher's Brook is undoubtedly a positive step forward," said RSPCA equine consultant David Muir. "However the RSPCA remains concerned about drop fences."
The modifications follow an analysis of all races run on the Grand National course since 1990 and consultations with animal welfare groups, the Royal Society for the Protection of Animals and World Horse Welfare.
Three horses died in the 1998 race
the opening day of the 2000 meeting was marred by four deaths.
Jamie Stier, director of Raceday Operations and Regulation for the British Horseracing Authority, said that lessons had been learned from such incidents.
"Aintree, our team of Course Inspectors and our Senior Veterinary Advisor have analysed DVD footage of races and fallers over the National Course since 2000," he said.
"I truly believe it all makes for a strong package of track changes that will enhance rider and equine welfare."
Tony Tyler, deputy chief executive of horse charity World Horse Welfare added: "Racing is never risk-free for horse or jockey, but by making these changes, Aintree is demonstrating that they do care about horse welfare and we hope that next year's race will be the safest yet."
The BHA will publish its full review of the 2011 race in October with the sport's governing body considering a post-race horse wash down for horses and shortening or removing the pre-race parade in the event of unseasonably warm wather.