Doctors treating Thailand's ailing King Bhumibol Adulyadej have performed a procedure to reduce a new build-up of water on the brain, the palace said on Sunday, the latest in a series of procedures for the 88-year-old.
Bhumibol is the object of an intense personality cult and his frail health is a matter of significant public concern.
The king is confined to a wheelchair and rarely seen in public.
He has spent most of the past two years hospitalised in Bangkok for a series of ailments, including bacterial infections, breathing difficulties, heart problems and hydrocephalus (water on the brain).
The latest health update released by the palace on Sunday said doctors had to install a new catheter to drain excess fluid in his brain.
"An x-ray examination on 14 June 2016 found his majesty had more fluid on his brain," the Royal Household Bureau said.
The statement added that the procedure, which was done without anesthetic, was successful.
Hydrocephalus is a potentially fatal condition where there is an excess of cerebrospinal fluid, a liquid which cushions the brain and brings it vital nutrients.
Previous health updates from the palace, including one last month, have mentioned the king being treated for hydrocephalus.
Earlier this month Bhumibol also underwent an operation to widen arteries in his heart. Sunday's statement said tests showed his heart condition had improved since then.
Bhumibol is the world's longest-reigning monarch and most Thais have never known life under another king.
He is largely seen as a unifying force in a nation bitterly divided along political lines.
Anxiety over what will happen after his reign ends is considered an aggravating factor in the country's past decade of tumultuous politics, as competing elites jostle for power and influence before the transition.
Bhumibol has not been seen by the public since September when the palace released a video of him being taken in his wheelchair to visit a shop inside Bangkok's Siriraj hospital. Shoppers and attendants knelt and bowed as he passed by.
An official photograph of him attending a ceremony was released by the palace in December, while officials also released a statement in January saying he briefly left his hospital for a brief trip by car to visit a palace.