Fyn-tyrosine-kinase-deficient mice exhibit increased fearfulness. To elucidate the neural mechanisms of their emotional defects, we compared fyn(-/-) and fyn(+/-) mice by behavioral analysis of conditioned fear and by functional neuroanatomical analysis of the distribution of highly responsive neurons associated with conditioned fear. The mice were exposed to the auditory conditioned stimulus paired with electric shock as the unconditioned stimulus. After the fear conditioning, auditory stimulus-induced freezing behavior was enhanced in fyn(-/-) mice. When the occurrence of c-Fos-immunoreactive neurons in the brain of fear-conditioned mice was examined following exposure to the auditory stimulus, a significant increase in immunoreactive neurons was found in the amygdala, hypothalamus, and midbrain of both genotypes. The occurrence of conditioned-fear-dependent c-Fos-immunoreactive neurons was enhanced in the central, medial, cortical, and basomedial amygdaloid subdivisions, the hypothalamic nuclei, and the midbrain periaqueductal gray of the fyn(-/-) mice in comparison with the fyn(+/-) mice. However, remarkably, the occurrence of conditioned-fear-dependent c-Fos-immunoreactive neurons was very low in the basolateral and lateral amygdaloid subdivisions of the fyn(-/-) mice, in striking contrast to a significant increase in c-Fos-immunoreactive neurons in these subdivisions in the fyn(+/-) mice. These findings suggest that the increased excitability of the specific amygdaloid subdivisions including the central nucleus, and of the projection targets such as the hypothalamus and midbrain in fyn(-/-) mice, is directly related to the enhanced fear response, and that the decreased excitability in the basolateral and lateral amygdaloid subdivisions is involved in the defective control of the neural circuit for emotional expression in this mutant.