Verproside, a catalpol derivative iridoid glucoside isolated from Pseudolysimachion longifolium, is a candidate for anti-asthmatic drug. The dose-dependency of the pharmacokinetics of verproside was evaluated in rats after intravenous and oral administration. After intravenous administration of verproside (2, 5 and 10 mg/kg doses), the systemic clearance (Cl) was significantly reduced and AUC was significantly increased at 10 mg/kg dose compared to 2 and 5 mg/kg doses. The volume of distribution at steady state (V (ss)) remained unchanged as the dose was increased. The extent of urinary excretion was low for both intravenous (3.3-6.2%) and oral (0.01-0.04%) doses. Isovanilloylcatalpol was identified as a metabolite after intravenous administration of verproside and showed the significant decreases in AUC and C (max) at 10 mg/kg verproside dose. The reduced systemic clearance of verproside at high doses appears to be due to the saturable metabolism. Upon oral administration of verproside (20, 50 and 100 mg/kg doses), C (max) was nonlinearly increased. The extent of verproside recovered from the gastrointestinal tract at 24 h after oral administration was 0.01-0.72% for all three doses studied. The absolute oral bioavailability (F) was 0.3 and 0.5% for 50 and 100 mg/kg doses, respectively. Low F appears to be due to first-pass metabolism.