The syntheses of arachidonic acid and total fatty acids were measured in human placentas at the end of the first trimester and at term using 14C-acetate in in vitro incorporation experiments. The rates of arachidonic acid synthesis were determined to be 0.70 (first trimester) and 0.64 (term) mumol/h/100 g of placental tissue. The calculated rates of total fatty acid synthesis amount to 0.5 (first trimester) and 0.6 (term) mumol palmitate-equivalents/h/100 g. The results suggest that the human placenta seems to be not dependent on maternal arachidonic acid. Near term the placenta even could be source of arachidonic acid.