It has been suggested that the period of sleep onset is particularly suitable for making evoked response studies in infants and children. However, it has been shown in animals that sleep spindles which occur during this period interfere with the mediation of afferent impulses to the cortex. We therefore investigated somatosensory and auditory evoked responses in 12 babies aged 5–9 months whilst sleep spindles were present in the EEG and compared them to those obtained when spindles were absent. No consistent differences in shape, latency or amplitude of the evoked responses were found when spindles were present and when they were absent. Three possible explanations for this findings are discussend: 1. Primary sensory pathways are less important for cortical evoked responses in young children than they are in adults. 2. Spindles produced in thalamic centers besides the somasensory relay nuclei have obscured the effect of an occlusion occuring in the relay nuclei during spindle generation. 3. EEG spindles in early ontogenetic stages are produced in the cortex itself and are not dependent on thalamo-cortical pathways.