STUDY OBJECTIVES: To determine the scalp topography of the N300 response to stimuli of different modalities and to investigate the relationship of the N300 component to K-complexes and vertex sharp waves seen in the un-averaged EEG. DESIGN: Two experiments were conducted one using auditory; the other using respiratory occlusion stimuli presented during stage 2 sleep. Trials were classified on the basis of whether they produced a K-complex, a vertex sharp wave, or some other response. Auditory stimuli were presented in the form of an oddball paradigm, and averaged separately depending on whether they were "frequent" or "rare". In both experiments, responses were averaged separately based on the appearance of K-complexes, vertex sharps waves, or some "other" response to the stimuli. SETTING: Data were collected in the Melbourne University Sleep Laboratory. PARTICIPANTS: Young healthy male adults, eight in experiment 1 and six in experiment 2. INTERVENTIONS: NA. MEASUREMENTS AND RESULTS: Data were collected from 29 scalp sites. In all cases, N300 amplitude was maximal in the vertex sharp wave averages, despite being clearly present in the averages of K-complexes and "other" responses. The vertex maximal scalp topography of the N300 did not differ across response conditions or as a function of stimulus modality. This is consistent with the N300 being produced by the same intracranial generators in all cases. There were no effects of stimulus or response type on N300 latency. CONCLUSIONS: N300 should be viewed as a multi-modal component with a different underlying generator mechanism than that of the K-complex.