We assessed the effect of systemic heparinization on the in-vivo thrombogenicity of various micro- and guiding catheters in a swine model. Microcatheters were placed through 6-F guiding catheters into the common carotid arteries of swine for 30-min (short-term) and 90-min (medium-term) periods, with and without systemic heparinization. At the end of the placement period the microcatheters were retracted through the guiding catheters and fixed for scanning electron microscopy (SEM). Guiding catheters were harvested after 5 h placement, with and without systemic heparinization, by retraction through 8-F sheaths and fixed for SEM. The surfaces of both hydrophilic and nonhydrophilic microcatheters all demonstrated more accumulation of debris during placement without than with systemic heparinization. The difference was primarily in the amount of fibrillary material on the catheter surface. The guiding catheters also demonstrated increased debris accumulation without systemic heparinization. This suggests that, even when using relatively nonthrombogenic catheters, systemic heparinization is indicated during cerebral angiography.