The metabolic clearance rates (MCR) and secretion rates (SR) of immunoreactive gastric inhibitory polypeptide (IR-GIP) were determined in both fasted and fed rats by a specific radioimmunoassay (RIA). Rats were infused intravenously (iv) with porcine GIP dissolved in a blood replacement mixture at a constant rate of 0.12-0.13 ml/min for 60 min. The basal level of plasma GIP was decreased in fasted rats as compared to fed rats. The mean MCR of GIP was 1.85 ml/min in fasted rats and 1.96 ml/min in fed rats. There was no significant difference in MCR of GIP between fasted and fed rats. However, the SR was significantly higher in fed rats when compared with fasted rats. These results suggest that the low concentration of plasma GIP in fasted rats is due to a reduction of GIP secretion rate.