The first boy of the day, and fifth overall, was removed from the cave at 5 p.m., a source close to the operation told ABC News.
Around that time, an ambulance left the cave area and headed toward a helicopter, which landed at a nearby hospital. Medics appeared to remove one person on a stretcher but hid the person's identity behind multiple white umbrellas.
Three other ambulances have also been seen leaving the area, bringing the total for the day to four.
A heavy but brief downpour hit the area Monday morning, but authorities said that did not change the water level in the cave, as workers continue to pump water out. New oxygen tanks were being placed before the second stage of the rescue effort began.
Chiang Rai acting Gov. Narongsak Osatanakorn said they "hope to hear good news in the next few hours."
"All conditions are still as good as they were yesterday," Narongsak told a news conference. "The boys' strength, the plan -- today we are ready like before. And we will do it faster because we are afraid of the rain."
Four boys were retrieved from the cave Sunday, with Narongsak noting that the healthiest boys were removed first. The rescued children were taken to the hospital, and officials said early Monday that they are strong and safe but need to undergo detailed medical checks.
"This morning they said they were hungry and wanted to eat khao pad grapao," Narongsak said, referring to a Thai dish of meat fried with chili and basil and served over rice.
Thai authorities said the parents of the four rescued boys are still being kept from physical contact with them due to fear of infections. Relatives were able to see them through a glass partition.
Interior Minister Anupong Paojinda had said early Monday that the same divers who took part in Sunday's rescue would return to extricate the others as they know the cave conditions and what to do.
The boys and their coach had been stranded in Tham Luang Nang Non since June 23 when they went exploring in the cave after a scrimmage. Monsoon flooding cut off their escape and prevented rescuers from finding them for almost 10 days.
Authorities have said extracting the entire team from the cave could take up to four days, but Sunday's success raised hopes that it could be done faster.
The Associated Press contributed to this report.