By 1964, the stress of road travel, composing, producing and maintaining a high level of creativity became too much to bear for Brian Wilson. In December that year, while on a flight to Houston, Brian suffered from an anxiety attack and left the tour. Shortly afterward, he announced his withdrawal from touring to concentrate entirely on songwriting and record production. This wasn't the first time Brian had stopped touring. In 1963, when Jardine returned, Brian left the road; but when Marks quit, Brian had to return in his place. For the rest of 1964 and into 1965,Glen Campbell served as Wilson's replacement in concert, until his own career success required him to leave the group. Bruce Johnston was asked to locate a replacement for Campbell; having failed to find one, Johnston himself subsequently became a full-time member of the band, first replacing Wilson on the road and later contributing his own talents in the studio beginning with the sessions for "California Girls".
Jan and Dean, close friends with the band and their opening act 1963 and 1964, encouraged Brian to use session musicians in the studio. This, along with Brian's withdrawal from touring, permitted him to expand his role as a producer. Wilson also wrote "Surf City" for the Jan & Dean opening act. Their recording hit number one on the US charts in the summer of 1963, a development that pleased Brian but angered Murry, who felt his son had "given away" what should have been the Beach Boys' first chart-topper. A year later, The Beach Boys would notch their first number-one single with "I Get Around."
Soon, traces of Brian Wilson's increasing studio productivity and innovation were noticeable: "Drive-In," an album track from All Summer Long(1964) features bars of silence between two verses while "Denny's Drums", the last track on Shut Down, Vol. II (1964), is a two-minute drum solo. As Wilson's musical efforts became more ambitious, the group relied more on nimble session players, on tracks such as "I Get Around" and "When I Grow Up (To Be a Man)". "Help Me, Rhonda" became the band's second number-one single in the spring of 1965.
1965 led to greater experimentation behind the soundboard with Wilson. The album Today! featured less focus on guitars, more emphasis on keyboards and percussion, as well as volume experiments and increased lyrical maturity. Side A of the album was devoted to sunnier pop tunes, with darker ballads on the reverse side. This pattern was also evident on some of the band's singles; songs such as "Kiss Me, Baby" released on the B-side to "Help Me, Rhonda" and "Let Him Run Wild" on the B-side to "California Girls", each featured Brian Wilson on lead vocals, and foreshadowed the youthful angst that would later pervade Wilson's upcoming efforts.
In November 1965, the group followed up their number-three summer smash "California Girls" with another top 20 single, "The Little Girl I Once Knew". It is considered to be the band's most experimental statement thus far, using silence as a pre-chorus, clashing keyboards, moody brass, and vocal tics. Perhaps too extreme an arrangement to go much higher than its modest number 20 peak, it was only the band's second single not to reach the top 10 since their 1962 breakthrough. In December they would score an unexpected number two hit (number three in the UK) with the single "Barbara Ann", which Capitol released as a single without input from any of The Beach Boys. A cover of a 1961 song by The Regents, it became one of The Beach Boys most recognized hits over the years.