Bursts of rhythmical fast waves (> 1 mV, peak frequency approximately 16 Hz; mean frequency approximately 20 Hz) are elicited in the olfactory bulb and pyriform cortex in waking or urethane-anesthetized rats (1.25 g/kg, i.p.) by olfactory stimulation with organic solvents (xylene, toluene, methyl methacrylate, oil of turpentine) or components of anal gland secretions of rat predators (2-propylthietane, weasel; trimethyl thiazoline, red fox). These waves are specifically related to olfaction since they: (a) are blocked when the nares are sealed; (b) are not elicited by non-olfactory stimuli; (c) are unrelated to concurrent motor activity; and (d) can only be elicited in anesthetized-tracheotomized rats when an odorous airstream is drawn through the nasal passages. Pyriform fast waves appear to be somewhat specific to the odors of organic solvents and predators as other strong odors (ammonia, caproic and butyric acids, cadaverine) are ineffective. During natural sleep or after treatment with scopolamine hydrobromide, low voltage pyriform background activity is replaced by larger amplitude, irregular 1-20 Hz waves. The scopolamine-induced waves are not blocked by spontaneous motor activity. We suggest that the pyriform cortex, like the archicortex and the neocortex, receives a cholinergic activating input.