The in vivo activity of different 3-hydroxy-3-methylglutaryl coenzyme A (HMG CoA) reductase inhibitors (vastatins) on neointimal formation induced by insertion of a flexiblecollar around one carotid artery of normocholesterolemic rabbits was investigated. The contralateral carotid artery served as a sham control. Pravastatin, lovastatin, simvastatin, and fluvastatin were given mixed with food at daily doses of 20 ing/kg body wt for 2 weeks starting on the day of collar placement. The treatment with vastatins did not modify rabbit plasma cholesterol concentrations. The neointimal formation was assessed by measuring the cross-sectional thickness of intimal and medial tissues of fixed arteries with light microscopy. Fourteen days after collar placement, intimal hyperplasia (mostly cellular) was pronounced in treated carotid arteries. The intimal/medial (I/M) tissue ratio was 12-fold higher in treated arteries than in arteries without the collar (0.36±0.04 versus 0.03±0.02). Animals treated with lovastatin (R=12), simvastatin (n=12), and fluvastatin (R=12) showed significantly less neointimal formation; I/M tissue ratios were 0.24±0.03, 0.20±0.03, and 0.17±0.03, respectively. The inhibition elicited by pravastatin (n=12, 032 ±0.03) did not reach statistical significance. or-Actin antibody immunofluorescence analysis of serial sections revealed that cells present in the hyperplastic intima were mostly myocytes. Rates of intimal myocyte proliferation were also measured by incorporation of 5-bromo-2′-deoxyuridine, a thymidine analogue, into replicating DNA. Immunofluorescence analysis showed that 5-bromo-2′-deoxyuridine was actively incorporated into intimal myocytes after insertion of the collar, with a labeling index (percent of labeled myocytes) of 2.15 after 14 days. Labeling indexes for pravastatin-, lovastatin-, simvastatin-, and fluvastatin-treated carotid arteries were 2.01, 1.32,1.23, and 1.20, respectively, suggesting a direct effect of vastatins on arterial myocyte proliferation. The different responsiveness shown by the vastatins tested may be attributed to the differences in their capacity to penetrate cell membranes and their potency in inhibiting the HMG CoA reductase enzyme. We conclude that the inhibition of carotid intimal myocyte proliferation by these vastatins is independent of their effect on plasma cholesterol concentrations.