"It is very impressive. You see all sorts of details... they seem to stare at you from out of the darkness," said 39-year-old journalist Javier Ors after visiting.
"There is a life-size deer which looks like a female that is pregnant. That was impressive."
Another visitor, Andrea Vicente, said she was "very moved" by the experience.
"It gives you goose bumps," she said.
The five crept in wearing white masks and overalls and closed the door behind them as they headed underground to see the ancient masterpieces.
Experts arranged the tour as an experiment to assess the impact on the paintings from readmitting the public after 12 years of studies.
The culture ministry said scientists would monitor the temperature of the air and rocks, humidity, carbon dioxide and any risk of contamination, to see whether visits could safely continue.
- 'Earliest accomplished art' -
The highlight of the cave is a set of paintings, at least 14,000-year-old, of red and yellow bison plus horses, deer, humans with the heads of animals and mysterious symbols.
"When you enter, it is an extraordinary feeling," Gael de Guichen, the site's lead conservationist, told AFP.
"A prehistoric human saw a herd of bison on the plain -- some of them grazing, some sleeping,running or snorting. And he went back into the cave and painted it."
UNESCO listed the paintings as a World Heritage Site in 1985, as "masterpieces of creative genius and as humanity's earliest accomplished art".
The cave, whose walls are covered with colourful paintings over more than 270 metres (yards), was discovered in 1868 in northern Spain.
It has been dubbed the "Sistine chapel of Paleolithic art".
Experts say the cave was inhabited approximately 35,000 to 13,000 years ago.
The techniques of the paintings and realistic animal details mark "one of the key moments of the history of art", UNESCO says in its listing.
The caves are particularly well preserved and the style of paintings is unique to Cantabria, according to the world cultural heritage body.
During the closure, visitors have had to look at a replica of the paintings, with only scientists allowed into the cave to carry out research.
In January, the foundation which manages the cave said it could reopen but only to groups of five people a week, and for just minutes at a time.
Overall 192 visitors will be allowed in by August, when experts will reassess the impact of the visits on the paintings, the culture ministry said.