FK506 (Tacrolimus) is an effective immunosuppressant currently used worldwide in organ transplantation. Based on our recent findings that insulin-like growth factor-I (IGF-I) is important for the stimulation of choleresis in vivo, in this study we investigated the effect of FK506 on bile flow and the plasma and hepatic levels of IGF-I in rats. Intravenous treatment of rats with FK506 resulted in a significant increase in bile flow, whereas cyclosporin A induced a significant decrease. A significant increase in plasma levels of IGF-I was observed in rats 30 min after a single intravenous administration of FK506. Oral treatment of rats with FK506 for 1 week also resulted in an increase in both plasma and hepatic levels of IGF-I. Overall, this study showed that FK506 treatment increased bile flow and also induced an increase in the plasma and hepatic levels of IGF-I in rats, suggesting that a stimulation of hepatic IGF-I production by FK506 may contribute to its choleretic profile.