The role of the thyroid in developmental changes in membrane fatty acids was examined in the rat. Three tissues were studied in euthyroid and hypothyroid rats, the tissues were liver, heart and brain. Hypothyroidism was acheived by providing pregnant and young rats with drinking water containing 0.05% PTU. Both euthyroid and hypothyroid rats were killed at 1, 5, 15 and 25 days of age as well as adult rats. The following parameters were measured in both groups ; plasma T^ and T3 concentrations, body weight, tissue weights, phospholipid content of tissues, fatty acid composition of phospholipids isolated from these tissues
