This paper reports a comparative study of the influence of dietary fats on the delta 5 desaturation of dihomo-gamma-linolenic acid (20:3 n-6) to arachidonic acid (20:4 n-6) using rat liver microsomes. The kinetic conditions needed to measure the specific activity of desaturation have been described. The rate of delta 5 desaturation was studied at two substrate levels. In a first series of experiments, the liver microsomes of animals maintained on a chow diet showed less desaturation activity than those of animals fed a fat-free diet. For longer periods of control (fat-free) diet time, the amount of 20:4 n-6 produced in vitro was depressed and the 20:3 n-9/20:4 n-6 ratio in total lipids was enhanced. In a second series of experiments, groups of rats were fed semi-synthetic diets which contained 5 or 10% of dietary fats with different percentages of linoleic acid (18:2 n-6), alpha-linolenic acid (18:3 n-3), oleic acid (18:1 n-9) or saturated fatty acids. Liver microsomal delta 5 desaturase activities that were generally depressed, when compared with control rats, were moderately affected by dietary fats, except with a diet containing saturated fatty acids or alpha-linolenic acid for which delta 5 desaturation was decreased. These observations are discussed in relation with the recent literature.