Emergency services were called to the Virgin Active gym, in Crawley Leisure Park, after a man in his 30s suffered a cardiac arrest.
Paramedics were called to the gym just before 9pm.
A spokesman for the South East Coast Ambulance Service said: “He was not breathing. We sent crews in two cars and an ambulance and attempted resuscitation at the scene.
“The treatment was carried on en route to the Royal Sussex County Hospital.
“He was in a very serious condition.”
Firefighters from nearby Crawley fire station also attended.