By late 1983, the band experienced its first significant evolution when Kerry King, Slayer's guitarist, temporarily joined as second guitarist, replacing Greg Handevidt for live performances. This collaboration between two of the future most important Californian thrash metal bands demonstrated the early fraternity of the underground movement.
The lineup during this period featured Dave Mustaine (vocals and rhythm guitar), David Ellefson (bass), Kerry King (lead guitar), and Lee Rausch on drums. Lee Rausch represented a crucial advancement by becoming the band's first truly stable drummer, providing the rhythmic foundation necessary for Megadeth to begin playing regularly at Los Angeles clubs like the Whisky a Go Go and the Troubadour.
This lineup allowed the band to develop their stage presence and gain experience in the Californian underground circuit, establishing the first connections with the emerging thrash scene that included bands like Metallica, Slayer, and Exodus.
