In order to estimate the synthesis of arachidonate in the liver of fetal, suckling and pregnant rats, the conversion rate of linoleate to γ-linolenate was studied. The significantly higher conversion rate was found both in fetal and pregnant animal, whereas the ratio of arachidonate to linoleate in plasma lipid as well as in liver microsomal lipid was not elevated in pregnant rats. Instead of reduced conversion rate of 6-day suckling rats, the ratio of arachidonate to linoleate in plasma was higher than that of pregnant animals, probably due toincreased supply of linoleate as substrate from mother’s milk.