ABSTRACT. Emotional behavior can be characterized by interactions between genes and environments, especially in the developmental period. However, in mice, the effects of paternal care on offspring's emotional development are not well understood even in cases in which the male mice show stable paternal care to the pups. In this study, we used a reciprocal hybrid model of C57BL/6 and BALB/c mice, which show different parental behaviors and emotional responses, and compared the emotional responses in pups in their adulthood by using the elevated plus-maze test. It was found that inbred male mice showed intensive parental behavior toward their pups, especially as parental contact behavior, and that the licking/grooming behavior of males was dependent on the strain of the female they were paired with. Also, BALB/c pairs showed less parental contact behavior than C57BL/6 pairs. In the elevated plus-maze test, BALB/c males and females showed higher anxiety behavior, and reciprocal hybrid offspring showed intermediate values between C57BL/6 and BALB/ c mice. Although no significant difference was found in the parental care that offspring received and anxiety-related behavior between the two reciprocal hybrid strains, BALB/c pairs showed less parental contact behavior toward the pups as compared to C57BL/6 pairs, which would be associated with the higher anxiety-related behavior exhibited in the adulthood of BALB/c strain.KEY WORDS: anxiety, maternal care, mice, paternal care, reciprocal F1 hybrid.J. Vet. Med. Sci. 72(12): 1589-1596, 2010 Social environments during the early stages of life, in addition to genetic variations, have long-lasting influences on the behavioral development of individuals. The motherpup interaction has especially strong influences on the behavioral characteristics of rodent offspring in their adulthood [6,26]. For example, anxiety and maternal behavior are associated with maternal care offspring received in their infant period [8,27]. Another thing is that maternally deprived pups show persistent stress-endocrine response, suggesting deficiencies in their negative feedback system [32]. We have shown that early weaned mice and rats deprived of maternal care in the neonatal period as well as in the postnatal period of day 15 to day 21 display reduced maternal behavior [23] and increased anxiety-related behavior [20,21]. Classic studies in mice kept in semi-natural environments showed bi-parental care and monogamous breeding colonies [9, 31], indicating that not only maternal care, but also paternal care comprises important social stimuli for the developing pups. It is unfortunate that the effects of paternal care on the behavioral development of offspring have not been well documented.The genetic influences on behavioral development have been well demonstrated. Some studies have assessed genomic influences on behavior using hybrid F1 mice. For example, offspring from different genetic strains showed different levels of anxiety even though they received mothering from a mother of the same strain [...