Apprentice jockey Thomas Sadler has been taken to Royal Melbourne Hospital following a fall in the Sportingbet Powered By William Hill Handicap at Moonee Valley on Saturday.

Sadler was riding the $13 chance Massiyn when the imported galloper suffered a suspected heart attack leaving the home straight of the 2040-metre race.

The remaining 13 runners speared around Massiyn and luckily no others came to grief.

Sadler was lapsing in and out of consciousness before being transported to hospital.

But Victorian Jockey's Association chief executive Des O'Keefe said the young rider was later fully conscious, although complaining of lower back pain.

Sadler's father, Caulfield trainer John Sadler, said his son was initially conscious and alert.

"They've still got some concerns with him, but he seemed terrific straight after when I got over to him," Sadler said.

Fellow apprentice Chad Schofield, who won the race aboard $21 chance Schockemohle, said the fall had taken the gloss off the win.

"It was a horribly run race," Schofield said.

"You could see it unfolding at the bottom of the straight and it eventually happened.

"It's not good to see and unfortunately I ran over Tom and I'm not feeling too good right now."

As the fall happened turning out of the straight after the winning post, a clerk of the course was dispatched and witches hats were put in place to make riders aware of the problem.

Stephen Baster, who rode the $4.40 favourite Magnapal, said jockeys were fully aware of the drama that had unfolded.

"Luckily more didn't come down," Baster said.

"We were all aware of what had happened and worried whether it might be a no-race."

Massiyn has been taken to Werribee Veterinary Clinic for a full autopsy to determine his cause of death.