The goal of the study is to investigate the contribution of delta and theta responses to N200 and P300 ERP components that are recorded from two topographical sites (Fz and Pt) under two experimental paradigms (mismatch negativity and oddball) that trigger different cognitive processes for the respective task performances. The present study was conducted on 42 normal young adults. The results showed that it is the 'interplay' between the theta and the delta oscillations that produces the morphology and the amplitude not only of the P300 but also the N200 component. The functional and physiological meaning of the delta and theta responses are discussed within the framework of the theory of oscillatory neural assemblies and the principle of superposition. (C) 2000 Elsevier Science Ireland Ltd. All rights reserved.