Jim Chanos, head of Kynikos Associates, spoke Wednesday at the Delivering Alpha conference in New York.
"These businesses are earning nothing," he said.
Chanos has previously disclosed his bets against the two companies, with his position against Mednax in place since mid-2017 and his Envision short going since the beginning of 2018.
Share prices for both dropped after Chanos presented his remarks.
Chanos has called the companies "rent seekers" in the health-care system and charged again Wednesday that they are shuffling money to cover up how poorly the businesses are performing.
"If nothing happens you're going to see cash flow declines unless of course they ... spend hundreds of millions of dollars each year to keep your cash flow afloat," he said. "They're not going bankrupt tomorrow. On the other hand, they need to keep financing themselves just to keep the illusion going."
Mednax is a physician services provider with a network of more than 3,600 medical care providers. Similarly, Envision is the nation's largest provider of physician staffing.
Private equity giant KKR has announced its intention to purchase Envision, though Chanos expressed doubt about the deal. Envision has undergone criticism for sending out large out-of-network bills.
Rival Mednax trades at a high multiple now and has its own issues, including a fraud complaint from Aetna that Mednax has contested.