Tackling super-fast ovals in ridiculously quick IndyCar machines is never easy, and when something goes wrong, the drivers know the hit is going to be pretty hefty.
Penske racer Castroneves lost the rear of his car mid-way through Turn 1, spinning out of control up the circuit and into the barrier.
His car briefly flew up onto two wheels with the impact, but he was swiftly checked, cleared and released from the infield medical centre.
Later in qualifying, Hunter-Reay lost the rear of his car as he exited a corner and spun into the wall with a heavy whack – also momentarily going up onto two wheels.
His car then drifted across the track and made contact with the inside barrier too.
IndyCar has confirmed that the Andretti racer was checked at the medical centre before being transported by ground to hospital for further evaluation.