The mouse olfactory system detects odorants and pheromones. It also detects predator odors that cause innate fear responses. This suggests that there are genetically defined neural circuits within the olfactory system that are involved in instinctive fear responses to specific olfactory stimuli. Olfactory cues that stimulate innate fear response can be detected in the olfactory epithelium (OE) and/or the vomeronasal organ (VNO). From the OE, signals travel through the main olfactory bulb to the olfactory cortex (OC), which comprises multiple areas. VNO signals travel through the accessory olfactory bulb to two specific parts of the amygdala ('vomeronasal amygdala' (VA)). To investigate olfactory circuits that underlie innate fear responses, we have focused on CRH (corticotropin releasing hormone) neurons in the hypothalamus, which serve as key regulators of physiological responses to fear. To identify neurons that transmit olfactory signals to CRH neurons, we have infected CRH neurons with a conditional 'tracer/reporter' pseudorabies virus that travels retrogradely through chains of connected neurons. In addition, we have developed a dual virus monosynaptic tracing system that enables us to identify neurons directly presynaptic to CRH neurons. Our studies indicate that neurons presynaptic to CRH neurons are present in multiple areas of the OC and VA. Comparison of results obtained with monosynaptic and polysynaptic viruses suggests that CRH neurons are directly connected to the VA, but indirectly connected to the OC via intermediates in other brain areas. Surprisingly, a strong regional bias was observed in one major OC area, the piriform cortex, and in one VA area, the medial amygdala (MEA). Virus-infected neurons were observed predominantly in the posterior quarter of piriform cortex and the posteroventral quadrant of MEA, suggesting that there is a biased organization of neurons involved in innate fear responses within these olfactory areas. These results provide new insights into olfactory circuits regulating fear responses, and show that our mono- and poly-synaptic viral tracing methods are powerful tools to investigate neural circuits upstream of specific neurons.