The effects of 2.5 or 5 mg diazepam (DIZ) on the sleep spindles were studied in 12 healthy male subjects. Polygraphic recordings and the state anxiety scale of the State Trait Anxiety Inventory (STAI) were made for 6 consecutive nights. An inert placebo was given on the first 3 nights and on the sixth night, and DIZ was administered on the fourth and fifth nights. DIZ produced increases in the amount of the slow and fast spindles in a dose-dependent manner. DIZ dose-dependently lowered the frequency of the fast spindles and elevated that of the slow spindles. Furthermore, the influence of DIZ on fast spindles was greater than that on slow spindles. DIZ decreased the state anxiety of the subjects in a dose-dependent manner. These results suggest that measuring the amount and the frequency of fast spindles could be a useful tool in predicting the anxiolytic effects of benzodiazepines. Copyright © 2001 S. Karger AG, Basel.