Equivalent dipole sources of two types of human sleep spindles (14 and 12 Hz) were investigated on five normal subjects. The present study showed that a sleep spindle can be represented by a single equivalent dipole. For both 14 and 12 Hz sleep spindles, the equivalent dipole sources were estimated near the thalamus. The orientation of the equivalent dipole of a 14 Hz sleep spindle was in the centro-parietal direction, while that of a 12 Hz sleep spindle was in the frontal direction. These results suggest that both types of sleep spindle activities are generated in the thalamus, and cortical de-arousal plays a modificatory role on their different topographical distributions.