Plasma levodopa decay curves appeared to be triexponential up to 6 hr after single intravenous injections of levodopa in rats, rabbits, and dogs. After pretreatment of each of these three species with benserazide (Ro-4-4602), a decarboxylase inhibitor, plasma levodopa concentrations declined biexponentially. Administration of benserazide decreased both total plasma clearance and apparent volume of distribution of levodopa, but did not change the rate constant of the terminal phase of the plasma levodopa decay curve. Thus, decarboxylase inhibition failed to alter the plasma half-life of levodopa.