According to our working hypothesis, the resonance properties of the brain systems play an important role in internal brain communications (e.g., Başar, 1992; Başar, Başar-Eroglu, Demiralp & Schürmann, 1992). It was assumed that evoked potentials (EPs) reflect brain resonance properties, showing enhancement, time and frequency-locking during the poststimulus period. All these phenomena might be referred to the spontaneous (intrinsic) EEG rhythms according to the excitability rule and the related concept of brain system response susceptibility: a brain system could react to internal or external stimuli producing those rhythms or frequency components, which have already been present in intrinsic or spontaneous activity (Başar, 1980). In order to test the hypothesis of response susceptibility, in the present paper we used an natural model--3-year-old children--to investigate how brain systems respond to external stimulation if their spontaneous rhythms are different in comparison to the spontaneous EEG rhythms in adults. For that purpose we used a combined time and frequency domain approach. The spectral characteristics of the spontaneous EEGs as well as the frequency components of auditory EPs elicited under identical auditory stimulation in 3-year-old children and adults aged 20-22 years were compared. Our observations support the hypothesis for response susceptibility; if in a given frequency channel the spontaneous brain rhythms are missing, they are also absent in the evoked and induced rhythmicities and vice versa: children at 3 years do not create alpha resonance upon sensory stimulation while they do not have developed EEG rhythms in the range of 8-15 Hz. Elicited under identical experimental conditions (auditory stimulation with fixed stimulus parameters) children and adult evoked rhythms differ. It was concluded that the AEPs recorded in 3-year-old children might be regarded mainly as a superposition of rhythmicities in delta and theta ranges. These rhythmicities are prolonged and delayed in comparison to the corresponding rhythms in adults.