In order to simplify the non-invasive quantitative evaluation of cerebral blood flow (CBF), we used computer simulation and adopted the pulse function as the input function of compartment model for calculting the mean retention fraction of 99mTc-ECD in brain from the time activity curves (n = 24). We found that there is a significant correlation between R2 (the retention fraction) from the 2-compartment model and R3 from the 3 compartment model with a correlation coefficient of 0.9836. The CBF by R2 is 29.48 +/- 1.51 ml/min per 100 g, the CBF by R3 is 29.10 +/- 1.48 ml/min per 100 g, and no significant difference between them is seen (P > 0.05, n = 24). These data indicate that the simplified two-compartment model for the mechanism of the uptake of 99mTc-ECD in brain is better. And we found that, compared with the entricular function, the pulse function may be used as a more appropriate input function for the model.