Background: Astrocytes are cells within the central nervous system which are activated in a wide spectrum of infections, and autoimmune and neurodegenerative diseases.In pathologic states, they produce inflammatory cytokines, chemokines, and nitric oxide (NO), and sometimes they induce apoptosis.Their protease-activated receptors (PARs) can be activated by proteases, e.g.thrombin and trypsin,which are important in brain inflammation.The current study aimed to investigate the effects of different concentrations of trypsin (1 to 100U/ml) on cultured astrocytes.Methods: In the present study, two-day rat infants’ brains were isolated and homogenized after meninges removal, then cultivated in DMEM+10% FBS medium. 10 days later, astrocytes were harvested and recultivated for more purification (up to 95%), using Immunocytochemistry method, in order to be employed for tests. They were affected by different concentrations of trypsin (1, 5, 10, 15, 0, 40, 60, 80, and 100 U/ml). To reveal the inflammation progress, NO concentrations (the Griess test) were assessed after 24 and 48 hours.Results:The results showed that trypsin concentration up to 20 U/ml caused a significant increase in NO, in a dose- dependent manner, on cultured astrocytes (P < 0.001). Trypsin 20 U/ml increased NO production fivefold the control group (P < 0.001). At higher concentrations than 20 U/ml, NO production diminished (P < 0.001). At 100 U/ml, NO production was less than the control group (P < 0.001).Conclusion: Inflammatory effects of trypsin 5-20 U/ml are probably due to the stimulation of astrocytes’ PAR-2 receptors and the increasing of the activation of NF-κB, PKC, MAPKs. Stimulation of astrocytes’ PAR-2 receptors causes an increase in iNOS activation which in turn leads to NO production. However, higher trypsin concentration possibly made astrocyte apoptosis; therefore, NO production diminished. These assumptions need t be further investigated.