The neonatal period is critical for seizure susceptibility and neocortical networks are central in infantile epilepsies. We report that application of 4-aminopyridine (4-AP) to immature (P6-P9) neocortical slices generates layer specific interictal seizures (IIS) that transform after recurrent seizures to ictal seizures (IS). During IIS, cell-attached recordings show action potentials in interneurons and pyramidal cells in L5/6 and interneurons but not pyramidal neurons in L2/3. However, L2/3 pyramidal neurons also fire during IS. Using single NMDA channel recordings for measuring the cell resting potential (Em), we show that transition from IIS to IS is associated with a gradual Em depolarization of L2/3 and L5/6 pyramidal neurons that enhances their excitability. Bumetanide, a NKCC1 co-transporter antagonist, inhibits generation of IIS and prevents their transformation to IS indicating the role excitatory GABA in epilepsies. Therefore, deep layer neurons are more susceptible to seizures than superficial ones. The initiating phase of seizures is characterized by IIS generated in L5/6 and supported by activation of both L5/6 interneurons and pyramidal cells. IIS propagate to L2/3 via activation of L2/3 interneurons but not pyramidal cells, which are mostly quiescent at this phase. In superficial layers, a persistent increase in excitability of pyramidal neurons due to Em depolarization is associated with a transition from largely confined GABAergic IIS to ictal events that entrain the entire neocortex.