To quantify the extraction fraction, E, for gadopentetate across tumor capillaries, R3230 adenocarcinomas were implanted in the mammary fat pads of seven rats. The value of E was determined by using a two-compartment tissue model in which the endothelial transfer coefficient, K(PS) (ml x min(-1) x cc(-1) of tissue), was estimated from the model fitted to changes in R1 relaxation time (deltaR1; s(-1)) measured by dynamic three-dimensional spoiled gradient recalled magnetic resonance imaging after injection of 0.1 mmol x kg(-1) of gadopentetate dimeglumine. The plasma flow rate through the tumor capillaries, Fp, (ml x min(-1) x g(-1) of tissue), was independently measured with fluorescent microspheres. E could be calculated by the relationship, E = K(PS)/Fp. The mean E for gadopentetate in the R3230 tumor was 0.197 +/- 0.118 with a range of 0.123-0.454. The relatively small mean value of E for gadopentetate allows a fair approximation of the permeability surface area product by K(PS) in this R3230 tumor model.