Pituitary and plasma arginine vasotocin (AVT) concentrations were examined in correlation with plasma mineral concentrations in the lamprey acclimated to freshwater (FW) and diluted seawater (SW). There were significant differences in plasma osmolality and plasma concentrations of sodium, chloride, calcium and magnesium between lampreys acclimated to FW and those acclimated to diluted SW. Although the plasma AVT concentration in the FW group was not significantly different from that in the diluted SW group, the pituitary AVT content in the FW group was significantly higher than that in the SW group. These results seem to show that synthesis of AVT is increased in FW lampreys.