Plasma total lipids, total cholesterol (cholesterol esters and free cholesterol) and oxysterol (mainly 7 beta-hydroxycholesterol (7 beta OH)) concentrations were significantly elevated in New Zealand rabbits fed a 2% cholesterol-containing diet with respect to controls fed the same diet without cholesterol. In addition, linoleic (18:2 n-6) and alpha-linolenic acid (18:3 n-3) plasma concentrations were significantly elevated in hypercholesterolemic rabbits, while concentrations of long-chain n-6 and n-3 derivatives were reduced. Studies in monocytic cell line THP-1 revealed that 7 beta OH markedly inhibited the conversion of 18:2 to 20:4 n-6 and of 18:3 to 22:6 n-3, indicating depression of the desaturation steps; in particular the inhibition was greater for the Delta 5 desaturation step. Furthermore, experiments of Real-Time PCR showed that 5-10 microM 7 beta OH decreased the Delta 5 gene expression. In conclusion, atherogenic oxysterols interfere with the production of long-chain polyunsaturated fatty acids from their precursors both in hypercholesterolemic rabbits and in cultured cells.