Theta rhythm in many brain structures characterizes wakefulness and desynchronized sleep in most subprimate mammalian brains. In close relation to behaviors, theta frequency and voltage undergo a fine modulation which may involve mobilization of dorsal raphe nucleus efferent pathways. In the present study we analyzed frequency modulation (through instantaneous frequency variation) of theta waves occurring in three cortical areas, in hippocampal CA1 and in the dorsal raphe nucleus of Wistar rats during normal wakefulness and after injection of the 5-HT 1a receptor agonist 8-OH-DPAT into the dorsal raphe. We demonstrated that in attentive states the variation of theta frequency among the above structures is highly congruent, whereas after 8-OH-DPAT injection, although regular signals are present, the variation is much more complex and shows no relation to behaviors. Such functional uncoupling after blockade demonstrates the influence of dorsal raphe nucleus efferent serotoninergic fibers on the organization of alertness, as evaluated by electro-oscillographic analysis. Electrical potentials from thalamic, olfactory and septal nuclei, hippocampus, amygdala, most cortical areas and many other brain structures oscillate regularly from 6 to 11 Hz as theta rhythm during wakefulness and desynchronized sleep in rats (1). Theta wave amplitude and frequency undergo fine modulation as a function of time and of the kind of behavior occurring in those states (2). Such modulation is followed by a spectrographic correlation between central structures that is higher in attentive wakefulness than in the relaxed state and narrowing of the oscillation band also occurs, disclosing its functional coupling to wakefulness (3,4).The serotoninergic raphe ascending projections are diffuse and reach almost all mammalian brain structures (5,6). The dorsal raphe

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