The influence of CGP 35348 (a GABA(B) receptor antagonist) on the sleep-waking cycle was studied in rats. The animals were injected i.p. at the beginning of the light period and the data expressed by 2-h periods and total duration (6 h). At 100 mg/kg, slow-wave sleep (SWS) was decreased during the 6-h recording with a peculiar decrease during the first 2 h. SWS was subdivided into three stages: slow-waves; spindles occurring as SWS deepens; and intermediate stage appearing prior to paradoxical sleep (PS). Only the slow-wave stage and intermediate stage were decreased. Waking was increased during the 6-h recording. It was subdivided into waking with hippocampal theta rhythm (psychomotor active waking) and waking without theta activity (quiet waking). Both were increased during the first 2 h. However, quiet waking was increased throughout the recording duration. At 300 mg/kg, SWS was decreased during the three 2-h periods. This decrease was principally related to a decrease of the slow-wave stage. PS was increased over the 6-h recording with a marked increase during the second 2-h period. Consequently, under the influence of the GABA(B) receptor antagonist, the SWS was decreased at the expense of behavioral stages with cortical low-voltage activity (waking and PS). GABAergic neurons are present in the mesopontine structures responsible for these two stages. We can conclude that endogenous GABA acting at the GABA(B) receptor level participates in the regulation of waking and PS.