One of us showed previously [Cuajungco and Lees (1998) Brain Res. 799, 188-129] that nitric oxide injected into the cerebrum in vivo causes zinc staining to appear in the somata of neurons and suggested that this staining of somata might be accompanied by a depletion (release) of zinc from axon terminals. In the present study, we confirm earlier results and report that there is a dramatic loss (apparent release) of histologically reactive zinc from the boutons of zinc-containing axons induced by infusion of nitric oxide into the brain in vivo. Rats were anesthetized with halothane and a cannula was inserted into the hippocampus. Either nitric oxide donor (spermineNONOate, 100 mM/2 microl) or control (spermine, 100 mM/2 l) was infused into the hippocampus or the cerebellar cortex. Two hours after infusion, N-(6-methoxy-8-quinolyl)-para-toluenesulfonamide (TSQ) staining for zinc in the brains revealed that sperminenitric oxide, but not control (spermine only) produced up to 95% depletion of zinc staining from the zinc-containing boutons. TSQ-positive neurons were also conspicuous throughout injection sites, in both the cerebral cortex and in the cerebellar cortex, where the Purkinje neurons were especially vivid, despite the scarcity of zinc-containing axonal boutons. It is suggested that the TSQ-stainable zinc in somata might represent intracellular stores mobilized from within or permeating extracellular stores.