The effects of plasma protein concentration (TP) on sevoflurane plasma protein/gas partition coefficient and of hematocrit values (Ht) on sevoflurane red cell/gas partition coefficient were examined in dogs. Furthermore, the effects of TP and inhalated nitrous oxide on sevoflurane MAC and sevoflurane concentration in blood at 1 MAC were studied in dogs during sevoflurane anesthesia. Snginificantly posotive correlations were noted in vitro between TP and plasma/gas partition coefficient, and between Ht and red cell/gas partition coefficient. However, the change in blood/gas partition coefficient appeared to be affected by TP. Sevoflurane MAC and seveflurane concentration in blood at 1 MAC rose in positive correlation with the rise of TP with or without nitrous oxide in combination, but there was no correlation with Ht. This seemed to be related to the predominancy of sevoflurane to dissolve into the plasma. The sevoflurane MAC and sevoflurane concentration in blood were significantly lower in the 33% nitrous oxide combined group than those in the oxygen alone group, but there was no significant difference between the 66% nitrous oxide combined group and the 33% nitrous oxide combined group. Furthermore, TP affected the sevoflurane MAC but not the nitrous oxide MAC.