Fischer 344 rats showed sex difference in the percentage of oleate in lipids of the omental adipose tissue (Thorling, E.B. and Overvad, K. (1994) Nutr. Res. 14, 569-576). The development of this difference was studied with respect to time in rats maintained on laboratory chow, from the age of 3 weeks to 20 weeks. From the age of 3 weeks to 5 weeks the percentage of oleate increased slightly in both sexes. From the age of 5 weeks to 13 weeks the percentage (mean ± S.D., n = 5) increased further in the female rats from 24.8 ± 0.6% to 27.6 ± 0.6%, whereas in the male rats it dropped from 22.9 ± 0.4% to 20.2 ± 0.6% at the same time points, respectively. In adult rats, age 20 weeks, the percentage of oleate was 28.6 ± 0.3% and 19.6 ± 0.5% for females and males, respectively. Castrated males partly maintained their juvenile level, being 21.1 ± 1.1% fourteen weeks after castration at the age of 6 weeks. Oestrogen injections twice a week to the castrated rats increased their oleate percentage within the same period to 23.4 ± 0.3%, partly reflecting the increase observed in the female rats. Stearic acid showed similar but less pronounced changes. The ratio oleic acid/stearic acid was constantly higher in the female than in the male rats, and this difference increased with age. The results of the present study suggest that these changes in percentage of oleate in adipose tissue lipids may partly have been caused by an effect of sex steroids on the delta-9-desaturase.