The effects of three vasodilators on the shape of the arterial pressure contour in the dog were investigated. A modified Windkessel model of the arterial vasculature was used as a guide in interpreting the observed contour changes. The pressure curve during diastole has been viewed as the transient response of the arterial system and can be fit accurately to the equation The parameters in this equation and peripheral resistance were used to define the components of the third order Windkessel model consisting of proximal (C1) and distal (C2) compliances, an inertance term (L), and peripheral resistance (R). This model describes the gross mechanical behavior seen in the arterial system during diastole. The arterial pressure pulse, mean pressure, and cardiac output responses to intravenous administration of sodium nitroprusside (NP), nitroglycerin (NTG), and hydralazine (H) were studied using computerized parameter estimating techniques to determine the ai parameters in the pressure equation and their relationship to the Windkessel components. All three drugs caused C1 to decrease, but not significantly. NTG increased inertance from 0.076 to 0.134 mm. Hg/ml./sec.2, with a p value < 0.05. NP increased C2 from 0.027 to 0.046 ml./mm. Hg. (p < 0.01); NTG also increased C2, from 0.030 to 0.060 ml./mm. Hg (p < 0.05). Hydralazine had no significant effect on C2, but it did reduce R from 3.10 to 2.16 mm. Hg/ml./sec. (p < 0.01). Neither NP nor NTG showed a significant decrease in R. Thus, the three vasodilators apparently have different vascular effects, as demonstrated by the pulse contour analysis. Such an analysis may be a useful way to evaluate vasodilator therapy.