Superoxide anion production in neutrophils plays an important role in the microbicidal defense system in the body. In this study, isolated rat neutrophils were stimulated experimentally and examined by electron microscopy to determine the site of superoxide production and its subsequent translocation during different cell stimulation time periods. Blood and peritoneal neutrophils were incubated for periods of 5, 10, and 15 min with phorbol 12-myristate 13-acetate (PMA), N-formyl-Met-Leu-Phe (fMLP), and combinations of PMA and cytochalasin B (CB) and fMLP and CB. Ultracytochemical detection of O 2 Οͺ was performed with the 3,3Π-diaminobenzidine-manganese (DAB/Mn) cytochemical method and cationized ferritin (CF) particles were added to stimulation media to monitor endocytotic events that occurred during neutrophil stimulation. Unstimulated neutrophils were devoid of O 2 Οͺ activity in cytoplasmic granules and at the plasma membrane surface. After 5 min stimulations with PMA, PMA Ο© CB, or fMLP Ο© CB, electron-dense DAB/Mn reaction product was detected in small, centrally located tubular compartments within the neutrophils. CF particles which were added to the stimulation media became internalized in endocytotic vesicles after 5 min stimulation; these vesicles were devoid of O 2 Οͺ activity. At 10 min stimulation with PMA, O 2 Οͺ -positive granules subsequently fused with each other and translocated to sub-plasma membrane regions where they either contacted the plasma membrane or fused with CF-containing endocytotic vesicles. Little reaction product was observed on the surface of the neutrophils. Spectrophotometric comparison of the stimulatory effects of PMA, fMLP, and fMLP Ο© CB revealed different rates and yields of O 2 Οͺ production. Results from this study suggest that the O 2 Οͺ -producing sites of rat neutrophils originate intracellularly and translocate to the plasma membrane surface following stimulation with PMA, PMA Ο© CB, and fMLP Ο© CB, but not with fMLP or CB alone. Furthermore, these compartments appear to possess the ability to fuse with endocytotic vesicles, a process that may be linked to intracellular microbicidal activity in circulating and tissue neutrophils.