We previously reported that the presence of a conspecific animal blocked freezing of a male rat in response to an auditory conditioned stimulus that had been paired with foot shocks, as well as associated Fos expression in the paraventricular nucleus. Here we investigated how this 'social buffering' is mediated by examining the contributions of both physical contact and the main olfactory system. Fear-conditioned rats exposed to the conditioned stimulus alone responded by freezing and increased Fos expression in the paraventricular nucleus. However, the presence of another rat, but not a guinea pig, dramatically mitigated these responses, even if the dyad was separated by a wire mesh or a pair of wire meshes 5 cm apart. In contrast, social buffering was absent when a transparent acrylic board was inserted between the double wire mesh. Lesioning of the main olfactory epithelium by injection of ZnSO(4) intranasally also abolished social buffering. Thus, we conclude that the main olfactory system is essential for the social buffering but does not require physical contact between the dyad.