Exclusive Details

Frank Sinatra Jr. died in a Daytona Beach hospital of a massive heart attack ... TMZ has learned.

Sinatra was in town to do a concert at the Peabody Auditorium on Wednesday night, but died suddenly in the afternoon ... according to his rep.

We're told Sinatra cancelled tonight's show when he felt lightheaded. He was admitted to Halifax Medical Center, but according to his rep he said he was feeling better. He suffered the heart attack while still at the hospital.

He was famously kidnapped in Lake Tahoe in 1963, when he was 19 years old ... and held ransom for $240,000. He was eventually found, safe and sound, walking the streets of Bel Air.

Reports dogged him for the rest of his life that the kidnapping was a hoax, but the FBI says it was real -- and the kidnappers were convicted.