The scoreline flattered the Spanish and European champions, who needed a brilliant performance from Brazil forward Neymar to come back from 1-0 down and see off a spirited Rayo side, who played some attractive attacking football at the Nou Camp.
Rayo's opening goal came after slack marking by fullback Dani Alves and centre backs Gerard Pique and Jeremy Mathieu looked amateurish at times in their attempts to repel the visitors' attacks.
Barca have already conceded 11 goals in eight matches this term compared with a mere 21 in 38 games last season as they swept to a fifth Spanish league title in seven years.
News that makes sense
Your trusted source for staying up-to-date with the world around you. Get free daily news updates and analysis, straight to your inbox.
"The bad run will pass I am sure, these are players with a lot of quality," Luis Enrique told a news conference after the victory at the Nou Camp.
"We are not focused enough and the first goal came after a mistake in concentration," added the former Barca and Spain midfielder.
"We gave the ball away a lot in the second half and Rayo stifled us well. We have to improve."
Barca's victory, with Neymar scoring four and creating one for Luis Suarez, put them level on 18 points with Real Madrid at the top, although Villarreal can reclaim first place with a win at home to Celta Vigo later on Sunday.
Barca's next outing is on Tuesday when they travel to BATE Borisov for their Champions League Group E clash.
(Writing by Iain Rogers; Editing by John O'Brien)
