A single-blind study was conducted in 13 right-handed normal male subjects to compare the effects of oral and i.v. papaverine on regional cerebral blood flow (rCBF). Six xenon-133 inhalation rCBF measurements were performed on each subject; three tests - baseline, placebo, and drug evaluations - were carried out on each of two separate days. The oral and i.v. drugs were randomized for first-day administration rCBF, measured as flow gray (FG), increased significantly (p≤0.001) from baseline with both drug forms. Increases of 10.53% and 13.94% (left and right hemispheres, respectively) were demonstrated 90 min after a single 600-mg dose of oral papaverine. Increases of 5.09% and 8.69%, repectively, were recorded immediately after a single 100-mg dose of i.v. papaverine. FG also increased significantly (p≤0.001) for both drug forms when compared to that of placebo. Placebo produced only a slight increase (not significant) with both the oral and i.v. groups. The data show that both oral and i.v. papaverine are equally effective in increasing rCBF in normal subjects.