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Polyunsaturated fatty acids are known to lower plasma cholesterol concentrations. We studied their effect on intestinal acyl-CoA: cholesterol acyltransferase (ACAT) activity in rats fed either salmon oil or corn oil (17% fat) with or without 1% cholesterol. After an 8-week feeding period we confirmed the hypolipidemic effect of salmon oil and we established its ability to stimulate ACAT activity in rats fed low-cholesterol diets. The most striking effect of 1% dietary cholesterol on ACAT activity was obtained in the control group (34% enhancement), whereas cholesterol supplementation had no effect on ACAT activity in the salmon oil group. The results enable us to suggest that n-3 fatty acids have an effect per se on ACAT activity; the regulation of enzyme activity by dietary cholesterol probably involves independent processes.