Three error sources which contribute to the total error of the second space derivative (SSD) used with the method of current source density analysis (CSD) were examined. The approximate formula usually used with CSD was the first error source. Another error originated from the finite interelectrode distance. The measurement error of the device with which the potential was recorded was the third error source herein considered. A relative SSD error was determined to estimate the effect of the errors examined. The experimental data (a section from the three-dimensional potential profile in annuran cerebellum produced by parallel fiber activation) were fitted by polynomials, Fourier expansion and cubic splines. Then a preferable interelectrode distance was obtained corresponding to the minimal relative error.