Leaders Real have a four-point advantage over champions Barca with five games left ahead of the clash at the Nou Camp and Ronaldo and Messi have already netted a Spanish record 41 times each in La Liga this season.
Portuguese Ronaldo, who set the previous best of 40 last term, has 53 goals in all competitions, while Argentine Messi, the man who replaced him as World Player of the Year and has won the award the past three years, has 63, including 14 in the Champions League.
He is only four short of the all-time record of 67 set by former Bayern Munich striker Gerd Mueller in the 1972-73 season.
The latest domestic showdown between the world's two richest clubs has an added significance in that Real and Barca could meet in next month's Champions League final as Real seek a 10th European crown and holders Barca chase a third triumph in four years and their fifth overall.
Both sides trail after their semi-final first legs, Real losing 2-1 at Bayern Munich and Barca falling 1-0 to Chelsea in London.
Spanish league champions the past three years, Barca have already beaten Real over two legs in both the Spanish Super Cup and the Copa del Rey this term and they came from a goal down to win 3-1 at the Bernabeu in La Liga in December.
Should they finish level on points, the title will be decided by head-to-head record not goal difference.
Their matches rarely pass without controversy, from accusations of refereeing bias to red cards to outright brawling. Jose Mourinho famously poked Barca assistant coach Tito Vilanova in the eye during a melee earlier this season.
With the combative Portuguese at the helm, hopes were high in Madrid that Real would be able to end Barca's domination, although he has yet to find the magic touch that brought him so much success in Portugal, England and Italy.
"It's going to be another enormously demanding game," Emilio Butragueno, a former Real and Spain striker now a club director, said after Tuesday's defeat at Bayern Munich.
"The result could affect how the remaining four league games go," he added.
"Barcelona will present the greatest possible challenge and I hope we can find inspiration."
Ronaldo versus Messi is just one of a host of match-ups in an historic rivalry that is played out on many levels.
Barca see themselves as the representatives of fiercely independent Catalonia, standing up to the hated central government of Madrid embodied in Real.
Battle is also joined in the Spanish sports press, with Madrid-based Marca and As and Barcelona's Sport and El Mundo Deportivo constantly sniping and antagonising.
In addition, the game pits Real coach Mourinho against the club where he was an assistant coach in the 1990s when current Barca boss Pep Guardiola was a player.
Mourinho, who has repeatedly suggested that Barca are given favourable treatment by referees, has only managed to get the better of Guardiola once in 10 games since joining Real from Inter Milan in 2010, a dramatic 1-0 extra-time success in last year's King's Cup final.
Barca knocked Real out of the Champions League in the semi-finals last season, when they also inflicted a 5-0 La Liga drubbing on Mourinho in his first trip to the Nou Camp as Real coach in November 2010.
"He (Guardiola) is an intelligent lad and he knows exactly how he has managed to win so many games," Mourinho said pointedly at a news conference after the Bayern game.
"He knows perfectly well how he has done it," added the Portuguese, who was fined by UEFA last season for accusing European soccer's governing body of helping Barca.
If either side wins Saturday's 'Clasico' they will have 87 victories in 219 official meetings since they first locked horns in the Copa del Rey in May 1902, compared with 86 for their opponent. There have been 46 draws.
Real are one goal away from breaking their own La Liga record for a season of 107 set under Welsh coach John Toshack and with Butragueno in the side in the 1989-90 season.
Spain's top two have again set a blistering pace this season, which no other club has been able to match for long.
Europa League semi-finalists Valencia are 29 points behind Barca in third ahead of their game at Real Betis on Sunday and Malaga, who play at Osasuna on Monday, are a point further back in fourth.