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.
Citadel’s Ken Griffin says the Trump tax plan's cap on property, state and local income tax deductions will hurt Northern states and high-end home values in those geographies.
Steve Bannon, former top advisor to President Donald Trump, talked to anchor and reporter Michelle Caruso-Cabrera at CNBC's Delivering Alpha event.
Bannon, the former White House chief strategist, was speaking at CNBC's Delivering Alpha.
His comments came from the Delivering Alpha conference in New York.
CNBC and Institutional Investor hosted the 7th Annual Delivering Alpha Conference on September 12, 2017.
Meet the 2017 advisory board of Delivering Alpha, which takes place on September 12, 2017 at The Pierre in New York City.
CNBC and Institutional Investor hosted the 6th Annual Delivering Alpha Conference on September 13, 2016.
CNBC and Institutional Investor are pleased to host the 8th Annual Delivering Alpha Conference in New York City on July 18, 2018. Please take note of the new month for 2018. Delivering Alpha continues to be an incomparable who’s who of the investor community with leading asset manager and institutional investors offering candid views along with illustrious political and economic commentators appearing in short segments moderated by CNBC talent and II editors.