&NA;The efficacy of U74006F in the prophylaxis of chronic cerebral vasospasm (VSP) was evaluated in a randomized, double‐blind, placebo‐controlled trial. Forty cynomolgus monkeys were divided by restricted randomization into 2 treatment groups of 20. Five animals from each treatment group were randomized into subgroups 1 and 2. The animals of subgroup 1 were studied pathologically. Brain biopsies of the animals in subgroup 2 were performed and studied with high‐performance liquid chromatography (HPLC). The remaining 20 animals supplemented the number studied angiographically. Significant VSP (P< 0.05) was detected in the majority of vessels from the clot side (right) of both treatment groups. Electron microscopy results showed positive correlation with the angiographic data. When comparing the effects of U74006F to those of the placebo at day 7, there was a significant difference (P< 0.05) in the degree of VSP in the right extradural internal carotid and right middle cerebral arteries. This resulted from a greater degree of VSP in placebo animals. Two animals developed delayed ischemic deficits, one from each group. The infarct of the U74006F animal was smaller than the infarct in the placebo animal. Although overall changes in phosphagen levels did not reach statistical significance, HPLC analysis of the cortical biopsies did show a decrease in the ATP/ADP±AMP ratio of 54% in placebo animals and only 7% in animals receiving U74006F. The middle cerebral arteries of 2 animals were also studied with HPLC. The vessels from the clot side had moderate angiographic VSP and the ATP/ADP+AMP ratio was reduced by at least 50% compared with the middle cerebral arteries of the contralateral hemisphere. There may be a role for the use of U74006F in the prophylaxis of VSP following subarachnoid hemorrhage. (Neurosurgery24:179‐186, 1989)