We studied sleep spindle activity in ten normal young subjects, five males and five females. The subjects were recorded on magnetic tapes according to standard procedures, and the tapes were scored automatically by a system described previously. Spindle activity was analyzed on the C4-T4 lead of electroencephalogram, using a bandpass filter and discrimination of the integrated output of the filter. Results indicated that spindle density was the same in stages 2, 3, and 4, and was much lower in rapid eye movement (REM) sleep. In stage 2, K potentials were less abundant than spindles and exhibited a much smaller individual variability. Whereas spindle density was very stable across two nights in the same subject, there was wide variation between subjects. In addition, there was a sex difference; female subjects presented more than twice as many spindles as males. The density of spindles during non-REM sleep was constant throughout the recording and did not show any cyclic variation.