Endocytosis at the site of fusion is infrequently observed. Chromaffin cells expressing VMAT-2 pHluorin were exposed to cycles of low, pH 5.5, and high, pH 7.4, every 10 s after an initial 15-s stimulation with 56 mM K⁺ in the presence of bafilomycin. (A) Two bright puncta (fusion events) that appear during stimulation are highlighted (circle and arrow). These events are subsequently insensitive to low pH exposure (quench) within 25 (circle) and 10 s (arrow). Bar, 1 µm. (B) Low pH-insensitive events (endocytic events) expressed as a percentage of total fusion events observed. (C) The initial intensity of all fusion events compared with the intensity of only the events that subsequently showed rapid endocytosis.