This study assessed sleep spindle activity and its relationship to transient EEG activation in young adult and aged cats. Sleep-wake variables were monitored polygraphically for 12 h in 5 young adult (2-4 years) and five aged (9-11 years) animals. Recordings were scored for behavioral state. Then, using bandpass frequency analysis, sensorimotor cortical spindles were evaluated in three, 5-min segments of the NREM sleep EEG. Both the incidence of transient arousals (TA) and spindle (much greater than 25 microV) densities were significantly higher in the aged animals than in the young adults. In the young animals only, spindle densities reliably predicted the incidence of TAs. We suggest that spindle expression varies in relation to ascending reticular activating system tone, constituting a functionally-inhibitory thalamocortical response to neurophysiological conditions which promote central activation.