By means of evoked potentials a direct efferent connection was found to run from the posterior hypothalmus and medial forebrain bundle to primary olfactory structures (olfactory bulb, olfactory tubercle and prepyriform cortex). The pathway from the hypothalmus to the olfactory bulb follows in the lateral olfactory tract at a conduction velocity 5-10 m/sec. The olfactory tubercle functions as a relay station for the efferent fibers from various sources, running to the olfactory bulb. In animals with electrodes chronically implanted in the olfactory structures, hypothalamic stimulation gives rise to a prolonged train of hypersynchronous bursts of activity (40-50 Hz), which resemble the arousal reaction. This response is modified by transecting the cervical sympathetic trunk. By pathways still to be defined, potentials are evoked in the olfactory bulb by stimulation of the cervical sympathetic trunk and the termination of these sympathetic fibers shows a common postsynaptic neuronal pool with axons of hyopothalmic origin. Epinephrine topically applied to the olfactory mucosa induced hypersynchronous activity in olfactory structures, quite similar to that consequent to hypothalmic stimulation. These results suggest a multichanneled hypothalmic modulation of olfactory input.