M icrocirculation was studied during 10 wk in untreated rabbits (n = 13) and in rabbits treated with dietary addition of 1% cholesterol (n = 13), 1% cholesterol + 1% of the antioxidant BHT (butylated hydroxytoluene) (n = 11), or 1% BHT (n = 5). The studies were performed by direct intravital microscopic imaging of the left and right conjunctivae with the use of a stereo microscope and a high resolution television camera. Microvessel diameter, erythrocyte flow velocity, and microhemorheologic conditions were evaluated quantitatively via a computer-assisted digital image processing system. Significant and marked changes occurred in all the above variables as a consequence of cholesterol feeding. After 3 wk of feeding there was a dramatic decrease (approximately 30%) in blood flow velocity in arterioli of the third order (P < 0.0001), accompanied by aggregation of cells in 40-50% of the smaller conjunctival vessels (P < 0.0001). These changes were enhanced further during the following 7 wk of treatment. All the above changes in the microcirculation were markedly reduced by the addition of BHT treatment. The diameter of the above arterioli decreased in the purely cholesterol-fed group (P < 0.005), whereas this did not occur in the group fed both cholesterol and BHT. In rabbits fed BHT in the absence of cholesterol, there was no significant effect on any assessed microcirculatory variable. In conclusion, the results demonstrate that the antioxidant BHT prevented early cholesterol-induced microcirculatory changes. This prevention occurred in the absence of a reduction of blood lipid levels. The results provide strong support for the hypothesis that a considerable part of the effects on microcirculation in hypercholesterolemia may be due to cholesterol-induced oxidations and not to cholesterol itself. The results are discussed in relation to the previously demonstrated antiatherogenic effect of BHT and the possible use of antioxidants in the therapy and prophylaxis of atherosclerosis.