Plant growth regulators hold a strong influence on the alternate bearing, despite the occurrence of many other factors. In the present research, alterations in endogenous GAs (gibberellins) and Abscisic acid-metabolites (ABA) were analyzed in different physiological periods and organs along with the role of flower bud abscission (Alternate bearing) in Pistachio (Pistacia vera L.) by high-performance liquid chromatography-electrospray ionization tandem mass spectrometry (HPLC-ESI-MS/MS). The results showed significant differences in the content of ABA and GAs between 'On' and 'Off' years in various tissues of pistachio cultivar. Nine GAs and seven ABA were identified in pistachio, namely, GA(3), GA(4), GA(7), GA(8), GA(19), GA(29), GA(34), GA(44), and GA(53); ABA, DPA, ABAGE, PA, 7'OH-ABA, neo-PA, t-ABA, respectively. Especially, DPA and GA19 were the dominant ABA and GAs analyzed in this work, respectively. The different GA and ABA-metabolites were found more in flower buds than in other organs. The maximum ABA and GAs peaks were obtained after 35 days of full blooming during May. Almost all ABA metabolites, as well as GA(19) and GA(44), increased during fruit kernel development at 55 days after flowering. The data of present research support that endogenous GAs and ABA produced in different organs influence pistachio flower bud abscission and, therefore, are closely related to alternate bearing.