This study examines the influence of lesions in the centralis superior raphe nucleus (CeSR) and adjacent paramedial pontine tegmentum on the sleep/wakefulness cycle (SWC) in cats. Sixteen cats had electrodes implanted for electro-oculogram (EOG), electromyogram (EMG), electroencephalogram (EEG) and ponto-geniculo-occipital (PGO) recordings. There were 10 experimental animals: seven animals received diathermocoagulation lesions destroying between 7 and 27% of the CeSR; the remaining three cats suffered bilateral lesions in the paramedial portion of the reticularis pontis oralis (RPO) and caudalis (RPC) nuclei. Six sham-operated animals were used as controls. Recordings were taken of all animals in continuous 23-h sessions once a week for 12 weeks. Results indicated that the threshold for SWC state changes (increase of wakefulness (W) and drowsiness (D), and decrease of slow wave sleep (SWS)) after CeSR lesion is approximately 11.3% following volumetric destruction of the nucleus. The amount of CeSR damage (CeSR-D) only correlated significantly with the amount of W (positive correlation) and SWS (negative correlation) during the first week post-lesion. The changes in W over the course of the study were different in the two experimental groups. In both groups, total W was increased with respect to the controls, however, these increases were observed earlier in the CeSR-D group. The return to near control values in SWC state over days 15-28 of the study does not represent a definitive recovery by the CeSR-D cats. All the SWC states returned to control values by the tenth week in the cats with paramedial reticular pontine damage.