To clarify the reason(s) why sika deer, Cervus nippon, on Mt. Ohdaigahara strip bark, we examined two hypotheses: (1) bark improves rumen digestion in summer; and (2) deer eat bark to obtain minerals in summer to remedy mineral imbalances. Rumen fluid and blood plasma of sika deer on Mt. Ohdaigahara were collected from culled deer for in vitro experiments and blood plasma analyses. The in vitro digestibilities of Sasa nipponica leaves (the primary forage of sika deer on Mt. Ohdaigahara), Abies homolepis bark, and a 20 : 1 mixture of S. nipponica leaves and A. homolepis bark were examined to clarify the effects of bark presence on digestion of S. nipponica leaves. The in vitro digestibilities of S. nipponica leaves did not change with season. The in vitro digestibilities of S. nipponica leaves in the mixture were not significantly different from those of S. nipponica leaves. The in vitro digestibilities of bark changed seasonally, and were negative in spring. This phenomenon might be caused by the different chemical and microbiological conditions of rumen fluid. Deer blood plasma profiles were analyzed to clarify their mineral levels. The K content in summer was significantly higher than in other seasons, probably be cause the K content of S. nipponica leaves peaks in summer. The blood plasma profiles of other minerals were normal, suggesting that hypothesis (2) is incorrect.