Spectral analysis of the background activity of the EEG was performed in 295 healthy children, 4--11 years old, using the relative powers of theta (3.5--7.0 c/sec), slow alpha (7.5--9.0 c/sec) and fast alpha (9.5--12.5 c/sec) bands. Non-parametric regression functions of spectral parameters were estimated using Rosenblatt's method. The correlation with age is best represented by the logarithmically transformed value of fast alpha activity in the occipital leads (r = 0.5783 for O2 and 0.5657 for O1). Differences between corresponding points of the two hemispheres are small. The fast alpha activity is slightly more prominent in central leads up to 8 years of age but becomes dominant occipitally after that age. In the maturation of the EEG there are significant differences between boys and girls: the mean percentage of slower activity is higher, and of faster activity is lower, in girls than boys. Thus the EEG maturation in girls up to 11 years seems to be retarded compared with boys.