May was driving the race favourite A G's White Socks in the Central Otago Cup at 4:11pm before about 5000 people yesterday.
Spectators in Omakau were shocked when May fell onto the track during the race.
He was taken by helicopter to Dunedin Hospital and the remainder of the day's event was cancelled.
On Friday, a hospital spokesperson said that he was now in a serious but stable condition in the intensive care unit.
Fellow racer and pundit Andrew Fitzgerald tweeted that the May family have confirmed he's in a stable condition and they aren't yet sure what caused the medical event or what it was.
Racing journalist Michael Guerin said May appeared to have collapsed in the sulky while leading the main race at Omakau yesterday afternoon.
Guerin said that a helicopter landed on the track to take May to hospital.
The chairman of the stewards in the Racing Integrity Unit, Vinny Munro, had confirmed May suffered a medical event. He said all races at Omakau have been abandoned.
Witnesses said he appeared to fall back in the sulky while the horse continued to race, then fall onto the track.
A witness at the track said race officials rushed to his aid.
Afterwards, many people who went to cash in their tickets stayed to donate the money to St John's Ambulance or May's family.
May has driven almost 3000 race winners in New Zealand - making him the third most successful driver in New Zealand trotting history.
He is a seven-time winner of the NZ Trotting Cup, New Zealand's biggest annual harness race.