The vomeronasal organs, the receptor organs of the accessory olfactory system, are important in chemical communication. Each organ contains receptor neurons sequestered inside a blind-ending tube with a narrow access duct. Large blood vessels surrounding the vomeronasal lumen act as a pump to draw substances into the lumen, under the control of vasomotor fibers in the nasopalatine nerve. Stimulation of the superior cervical sympathetic ganglion or nasopalatine nerve operates the pump (24) but its schedule of activation in awake behaving animals is unknown. Electrodes, implanted inside the vomeronasal organ capsule of male hamsters, recorded changes in electrical properties accompanying vomeronasal pump activation. Recorded signals were validated by anesthetizing the animals and recording from the same electrodes while driving the pump by nasopalatine nerve stimulation. Recordings in awake behaving animals show that the pump does not operate only in situations where the vomeronasal organ is known to be important. It appears to operate in response to any novel situation where the animal's attention is attracted. The signals recorded suggest that blood vessels are constricted repetitively by bursts of activity in the vasomotor sympathetic nerves each time the pump is triggered, while the underlying level of arousal is reflected in the ongoing sympathetic tone. The low selectivity in operation of the pump may require a greater degree of selectivity in the receptors than previously thought. The ready activation of the pump also suggests that the vomeronasal system may have other functions than the communication of reproductive events.