Ligation of the main femoral artery is followed by a rapid development of collateral arteries. To detect an increase in vascular permeability of the collateral arteries, colloidal carbon (1 ml/Kg, iv) was injected into 153 rabbits at various times after ligation or sham-ligation of the main femoral artery. These animals were killed later and collateral arteries was dissected. Increase in vascular permeability was identified by the presence of visible carbon deposits on the vascular walls. The carbon deposits were frequently found on the intermediate segments of the collateral system in legs with ligation, but were not found on any arteries either in legs with sham-ligation or in contralateral untouched legs. The increase in vascular permeability was observed in 15 of 17 animals as early as 6 hours, in 32 of 36 animals 1 day but only in 5 of 21 animals 6 to 8 days after ligation. The arterial segments with carbon deposits showed various structural changes such as fragmentation of the internal elastic layer, degeneration of the medial muscle and deposition of fibrinoid material. These results indicate that vascular permeability transiently increases in the intermediate segments of developing collateral arteries. Acute vasodilatation may be responsible for the increase in vascular permeability.