The aim of this study is to find out the possible trigger role of lipid peroxidation in vasospasm. Haemoglobin-free washed erythrocyte membranes (erythrocyte ghost) corresponding to 2.5 mg membrane protein was mixed with 0.5 umol NADPH 3 umol ADP, 4 umol ferrous sulphate and injected into the cisterna magna of cats to stimulate lipid peroxidation in vivo (Group 1). The second group was injected lml/kg whole blood and the control group 1 ml/kg saline. Angiographic studies revealed significant vasospasm in five cats in group 1. Severe vasospasm was seen in six cats in group 2. A significant increase in lipid peroxidation was observed in groups 1 and 2, compared to the control group. These results may suggest that free radical products may play an important role in the complex genesis of vasospasm in subarachnoid haemorrhage.