Jefferson County Deputy Coroner Jim Wesley said the initial post-mortem found no anatomical cause of death. Results of toxicology tests to determine whether drugs or alcohol contributed to Baze's death won't be available for about three weeks, he said.
Baze, 24, was found inside his sport utility vehicle in the stable area of the famed track that hosted the Kentucky Derby Saturday.
City police said foul play did not appear to be a factor, The (Louisville) Courier-Journal reported.
Baze took Derby second-place finisher Nehro to a second in the Louisiana Derby before Cory Nakatani took over the steed for the Arkansas and Kentucky derbies.
The newspaper said Baze last rode April 29.
Baze was a member of a prominent racing family that included his cousins, Russell, and Tyler Baze. Michael Baze had 918 career wins and $32.4 million in purses, the newspaper said.
Court records show Baze had been arrested in November and charged with possession of cocaine, The Courier-Journal said. He was to have appeared in Jefferson District Court Thursday for a preliminary hearing.