(1) Crude synaptosomal fractions (P2) derived from guinea-pig cerebral cortex were incubated in the presence of 50 mM KCl in a Krebs-glucose medium. Torpedo marmorata electric organs were stimulated electrically in vivo at 5 pulses/sec for 30 min by electrodes placed on the electric lobe. Synaptic vesicles were isolated from each source and the phospholipid compositions analysed and compared with vesicles from unstimulated controls. (2) Lysophosphatidylcholine was the only lysophosphoglyceride demonstrable in the synaptic vesicles from either source and its low levels did not increase as a result of chemical or electircal stimulation. In each case there was a close similarity of the phospholipid distributions in the vesicles taken from control and stimulated samples. (3) Control experiments indicated extensive decreases in the acetylcholine content of the vesicles from the stimulated electric organ and smaller decreases in the acetylcholine content of the synaptic vesicles from stimulated crude synaptosomal fractions. These fractions were found to respire linearly in the presence of 10 mM glucose and the vesicle fractions were shown to have low levels of contaiminating membranes as judged by marker enzyme analyses. (4) Crude synaptosomal fractions from guinea-pig cerebral cortex were incubated in a Krebs-glucose medium with labelled fatty acids and [3H]glucose in the presence or absence of 50 mM KCl. Subsynaptosomal fractionation was carried out and specific radioactivities of phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine and phosphatidylinositol were determined in fractions D (synaptic vesicles), E (microsomes) and H (disrupted synaptosomes). The release of neurotransmitter did not significantly enhance the labelling of phospholipids in any of the fractions studied as compared with phospholipids from unstimulated fractions. This was found after two incubation times and using [14C]oleate, [14C]arachidonate, [3H]palmitate and [3H]glucose.