We've already reported that nitrate tolerance upregulated the phosphodiesterase(PDE)1A1 gene expression. The present study investigated the time-phasic development of nitrate tolerance in the aspects of in vivo hemodynamic responses and PDE 1A1 gene expression. Development of In Vivo Nitrate Tolerance An osmotic minipump with either NTG or vehicle (propylene glycol) were implanted subcutaneously to Male Sprague-Dawley rats (250g to 300g). NTG was infused at an average rate of 10 µg/kg/min for 1, 2 and 3 days. Hemodynamic Responses to Subsequent Nitroglycerin Bolus Infusion To assess tolerance, rat mean arterial pressure (MAP) was continuously monitored by a catheter in the right common carotid artery using a polygraph recorder. Bolus doses of nitroglycerin of 1,10,100 and 300 µg/kg i.v. were infused through the right jugular vein. Quantitative Reverse Transcription-Polymerase Chain Reaction (RT-PCR) 18s rRNA was used as an internal control and PDE1A1 specific primers was used. Rat aortic VSMC (P7-12) at ≈70% confluence were growth-arrested for 48 hours prior to the drug treatment. We measured PDE1A1 mRNA levels treated with cGMP analogue (8-cpt-cGMP, 25uM) for the indicated time. Results NTG-treatment group revealed significant decreases in NTG-induced MAP drop compared to control values from 3.4짹2.0% ~ 26.8짹5.1% (1 day-NTG group), 3.9짹0.1% ~ 27.2 짹0.3 % (2 day-NTG group), 3.9짹0.7% ~ 17.5짹1.4% (3 day-NTG group) along with the NTG dose increment which indicated that changes in MAP were significantly blunted in the NTG-pretreated groups from the 1day after NTG treatment and remarkably blunted on day 3. mRNA levels of PDE1A1 in rat aorta according to the time periods of NTG-treated group were increased by 1.9짹0.2, 2.0짹0.3, 2.4짹0.2-fold respectively compared to control group. cGMP analogue increased PDE1A1 mRNA by 2.0 ~2.1-fold, suggesting that the chronic increase in cGMP stimulated by NTG treatment induces PDE1A1 gene expression, which subsequently hydrolyzes cGMP. We could know that the nitrate tolerance in the aspects of hemodynamic responses and PDE gene expression developed 1 day after treatment and continued thereafter.