Summary:The spindle activity of 10 healthy male adults, aged 20-24 years, was investigated over 7 consecutive nights. Spindle appearance showed a strikingly distinctive and reproducible pattern for each individual. There was no first-night effect and little night-to-night variation of spindle appearance rate per night. Eight male adults aged between 19 and 24 years were studied in an evaluation of methaqualone and flunitrazepam by an experimental schedule of 17 consecutive nights. Both drugs caused a significant increase of spindle activity expressed as appearance rate. By using spindle activity as a sleep parameter we could investigate in more detail the characteristics of hypnotics.