The accumulation of intracellular ionic zinc and pharmaceutical compounds, like the antibiotic sulfamethoxazole, may contribute to various neuropathologies. Sulfamethoxazole and the drug trimethoprim, are inhibitors of enzymes involved in the synthesis of tetrahydrofolate and also of carbonic anhydrases. The inhibition of the latter enzymes, which are localized both intra- and extracellularly and have a key role in pH regulation, causes alkalinization that is associated with higher spontaneous transmitter release. Intense synaptic stimulation causes the entry of released zinc into postsynaptic neurons, through glutamate receptor channels or voltage dependent calcium channels. The aim of this study was to evaluate the effect of sulfamethoxazole (180 μM) on basal postsynaptic zinc and to compare it with that caused by two depolarizing media, containing high potassium or tetraethylammonium, which may induce long term synaptic plasticity. The studies were performed in brain slices from gestating rats, at the mossy fiber synapses from hippocampal CA3 area, using the zinc indicator Newport Green. In the presence of KCl (20 mM) and sulfamethoxazole (180 μM) the zinc signals were enhanced, unlike in tetraethylammonium (25 mM). After sulfamethoxazole the tetraethylammonium evoked zinc signal had reduced amplitude. Thus, the data suggests that sulfamethoxazole enhances transmitter release affecting synaptic zinc physiology.