The impact of chemotherapy on longitudinal vascular changes taking place during the growth of an animal tumor, R3230 AC adenocarcinoma, was investigated. Two contrast agents of different molecular weights, gadolinium-diethylene-triamine-pentaacetic acid (Gd-DTPA; < 1 kD) and gadomer-17 (35 kD), were used in the dynamic imaging studies. Enhancement kinetics were analyzed by a pharmacokinetic model to derive parameters related to vascular volume and permeability on a pixel-by-pixel basis. Responders and non-responders were separated according to tumor size 10 days after the therapy. Changes in the vascular volume measured by gadomer-17 at 4 days after therapy revealed a clear distinction between the controls and the responders/non-responders. Mean vascular volume decreased by 42% in responders but was not significantly changed in the controls. The one non-responder had increased vascular volume after chemotherapy. Enhancement kinetics of gadomer-17 detected the changes earlier and with greater sensitivity than Gd-DTPA. In the control group, vascular permeability determined by gadomer-17 correlated with the longitudinal growth rates of tumors, suggesting that vascular permeability assessed by gadomer-17 could potentially serve as an indicator of aggressive tumor growth.