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1. Rats were given diets containing (% dietary energy): 46 arachis oil (AO), 36 partially-hydrogenated arachis oil (HAO)+ 10 AO, 36 partially-hydrogenated marine oil (HMO)+ 10 AO, or 46 of a combination of rape-seed oils high and low in erucic acid (RSO+LERSO).2. In the liver microsomes the content of arachidonic acid (20:4w6) was reduced in the groups given H A 0 + A 0and HMO + AO.3. The rates of A6-desaturation of linoleic acid into y-linolenic acid (18:306) and of ASdesaturation of dihomo-y-linolenic acid into arachidonic acid were studied in vitro at two substrate levels: a high substrate level reflecting maximal microsomal desaturase activity in rat liver and a low substrate level reflecting desaturase activity under physiological conditions. 4. Dietary HAO, rich in 18: 1 isomers, suppressed the A6-desaturase activity but not the AS-desaturase activity. Dietary HMO, rich in 18: 1, 20: 1 and 22: 1 isomers, reduced both A6-and AS-desaturase activities.

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