The effect of diets with various (n-6):(n-3) polyunsaturated fatty acid (PUFA) ratios and a constant polyunsaturated: saturated fatty acid ratio on the basal activity of chylomicron lipid synthesizing enzymes was investigated in rat intestinal microsomes. Enzymes studied were: acyl-CoA:cholesterol acyltransferase (ACAT); acyl-CoA:lysophosphatidylcholine acyltransferase (MGAT) and acyl-CoA:1,2-diacylglycerol acyltransferase (DGAT). Results showed that after a 4-wk feeding period, ACAT, ACLAT and DGAT basal activities were significantly enhanced by the dietary fatty acids of the (n-3) series, whereas MGAT activity was not affected. When the highest (n-6):(n-3) ratio (51.0) was compared with the lowest (0.8), the increase attained was about 58, 76 and 73% for ACAT, ACLAT and DGAT, respectively. Fatty acid composition of microsomal lipids was drastically altered by the diets because (n-3) PUFA replaced the (n-6) series as the dietary (n-6):(n-3) ratio was lowered. These compositional changes could explain the observed modification in the membrane-bound enzyme activities. We suggest that (n-3) PUFA ingestion leads to an enhanced velocity of chylomicron synthesis in rats.