Mortality following permanent occlusion of both common carotid arteries decreases as the time between the first and second occlusions increases in Fischer 344 rats. Our goal was to examine the possibility that collaterals develop after unilateral carotid artery occlusion. During temporary occlusion of both carotid arteries in nine ketamine-anesthetized male rats, mean +/- SEM blood flow in both parietal cortices was 23 +/- 4% of the preocclusion (control) blood flow (120 +/- 7 ml/min/100 g) measured by laser Doppler flowmetry (p less than 0.05). After permanent occlusion of one carotid artery for either 1-2 days (n = 10) or 6 weeks (n = 7), mean +/- SEM blood flow was 16 +/- 2% and 30 +/- 3% of control, respectively, during a temporary test occlusion of the other carotid artery. During the test occlusion, blood flow in the cortex ipsilateral to the 6-week occlusion was 170% that in the contralateral cortex (which was similar to the blood flow immediately after temporary occlusion of both carotid arteries) and twice that after 1-2 days of occlusion. Mean luminal diameter of the basilar-carotid anastomosis ipsilateral to the 6-week occlusion was 186% that of the contralateral anastomosis, which showed only minimal change, and 145% that after 1-2 days of occlusion. We conclude that during 6 weeks of permanent carotid artery occlusion the anastomosis enlarges and its blood flow or reserve increases. Thus, collaterals developed ipsilateral to, but not contralateral to, the 6-week carotid artery occlusion, which suggests the possibility of greater collateral protection on the permanently occluded side.