INVESTIGATIONS by Whittaker and Sheridan1 on permanganate fixed preparations of isolated synaptic vesicles have recently confirmed that the membrane of such organelles has the structure of a “unit membrane”2, but have also indicated that in this case the classical model should be corrected in order to take into account the results obtained on negative stained specimens which suggest a membrane structure consisting of two 40 A layers with an intermittent hydrophilic space between them.