The specific binding of radioiodinated concanavalin A (
125
I-con A) to rat brain synaptosomes was shown to be saturable. In the presence of excess con A binding was rapid and was completed within 5 min (
t
1/2
was 25 s) at 37°C, and at saturation the amount bound did not change over time. Under the electron microscope, concanavalin A-ferritin (con A-ft) bound to synaptosomes in two regions: in the extra-junctional plasma membrane and within the synaptic cleft of Gray type 1 and 2 synapses. Synaptosomes incubated with con A-ft at 37°C internalized bound lectin by endocytosis through coated pits. Endocytosis took place in the extra-junctional membrane, because it can occur before con A-ft has penetrated into the synaptic cleft, and continued for a considerable time (more than 30 min) after saturation of the receptor(s). Synaptic vesicles, which have at least two con A receptors on the internal aspect of their membranes, and cisternae, become labelled. When exocytosis was induced in synaptosomes by K
+
depolarizations, synaptic vesicle con A receptors became incorporated into the plasma membrane and were labelled with
125
I-con A causing a 2.5-fold increase in con A binding that was Ca
2+
dependent. These experiments thus provide evidence for the transient incorporation of synaptic vesicle membrane glycoproteins into the plasma membrane during transmitter release.