Phosphoinositide 3-kinase (PI3K) is one of the early-signaling molecules induced by growth factor (GF) receptor stimulation that are necessary for cell growth and cell cycle entry. PI3K activation occurs at two distinct time points during G₁ phase. The first peak is observed immediately following GF addition and the second in late G₁, before S phase entry. This second activity peak is essential for transition from G₁ to S phase; nonetheless, the mechanism by which this peak is induced and regulates S phase entry is poorly understood. Here, we show that activation of Ras and Tyr kinases is required for late-G₁ PI3K activation. Inhibition of late-G₁ PI3K activity results in low c-Myc and cyclin A expression, impaired Cdk2 activity, and reduced loading of MCM2 (minichromosome maintenance protein) onto chromatin. The primary consequence of inhibiting late-G₁ PI3K was c-Myc destabilization, as conditional activation of c-Myc in advanced G₁ as well as expression of a stable c-Myc mutant rescued all of these defects, restoring S phase entry. These results show that Tyr kinases and Ras cooperate to induce the second PI3K activity peak in G₁, which mediates initiation of DNA synthesis by inducing c-Myc stabilization.