Protein phosphorylation and dephosphorylation catalyzed by protein kinases and phosphatases respectively plays an important role in the regulation of cellular processes. Ser/Thr protein phosphatases can be classified into several subfamilies, but protein phosphatase 1 (PP1) and 2A (PP2A) enzymes are responsible for 85% of all the dephosphorylation events in eukaryotic cells. To study the physiological role of PP1, the investigation of novel specific PP1 inhibitors is crucial. Several PP1 inhibitors have been already described but their molecular mechanism and selectivity remained controversial. Recent studies have suggested by in vitro experiments that tellimagrandin I. (TG) and mahtabin A (MA) decrease the protein phosphatase 1 enzymes’ activity. Our goal was to study the physiological effect of these potential phosphatase inhibitors on synaptosomal-associated protein of 25 kDa (SNAP25), one of the potential neuronal substrate of these enzymes in B50 neuroblastoma cells and in mouse cortical synaptosomes. SNAP25 is one of the member of the SNARE (soluble N-ethylmaleimide sensitive factor attachment protein receptor) complex, along with synaptobrevin and syntaxin. The primary role of SNARE complex is to mediate vesicle fusion. Neither TG nor MA had an effect on the cell survival and apoptosis of B50 neuroblastoma cells. MA treatment of mouse cortical synaptosome resulted in an increase in the phosphorylation level of Thr138 of SNAP25, which is believed to be a regulatory phosphorylation site of the SNARE complex. MA also resulted in a decrease in neurotransmitter release using in vitro exocytosis assay of mouse cortical synaptosomes. However, TG treatment exhibited controversial results as the level of SNAP25 phosphorylation did not change significantly and the extent of exocytosis decreased only slightly. Our data suggest that MA but not TG is a potent and potential therapeutic PP1 inhibitor in neuronal cells.