Bryan Cranston has asked fans to pray and “send positive thoughts” to Bob Odenkirk after his former Breaking Bad co-star collapsed.
The 58-year-old was taken to hospital by ambulance after falling ill on Tuesday on the New Mexico set of Better Call Saul.
It is not clear what caused the collapse.
Cranston, who starred as drugs kingpin Walter White in Breaking Bad, shared a photo of them together on Instagram.
He wrote: “Today I woke up to news that has made me anxious all morning. My friend, Bob Odenkirk collapsed last night on the set of Better Call Saul.
“He is in the hospital in Albuquerque and receiving the medical attention he needs but his condition is not known to the public as yet.
“Please take a moment in your day today to think about him and send positive thoughts and prayers his way, thank you.”
Better Call Saul, the spin-off prequel to Breaking Bad, has been shooting its sixth and final season, which is due to air on US network AMC next year.
Like Breaking Bad, Better Call Saul is set in and mostly shot in Albuquerque, New Mexico’s largest city.
Odenkirk has been nominated for four Emmys for playing the title character, an unsuccessful lawyer named Jimmy McGill who becomes increasingly corrupt and greedy and later adopts the pseudonym Saul Goodman.