Prolonged-exposure (PE) therapy is the treatment of choice for posttraumatic stress disorder (PTSD). Despite its efficacy, a significant number of individuals will not benefit from it or might drop out before the completion of all sessions. This underlies the importance of finding ways to enhance the efficacy of PE in order to improve the life quality of individuals suffering from PTSD. It is now widely accepted that extinction learning paradigms used in fundamental studies are useful laboratory analogs to PE. Studies in healthy controls have suggested that elevated estrogen levels benefit extinction learning by promoting its consolidation and thus enhancing its recall when tested later for it. This is also being reflected by changes in the activation of brain regions forming the fear extinction network, including the amygdala, dorsal anterior cingulate cortex (dACC) and ventromedial prefrontal cortex (vmPFC). It is still unknown whether estradiol (E2) administration can modulate the activation of the fear extinction network in oral contraceptive (OC) users and which E2 dose could yield the best results. The R61 phase of the current study will aim to establish which of two E2 doses (placebo (Plc), 2mg or 4mg) can best engage the fear extinction network in healthy women using OC by exposing participants to a validated fear conditioning and extinction protocol. Functional fMRI (BOLD signal) and psychophysiological measures (skin conductance responses - SCR) will be used to test the following hypotheses: 1) E2 administration will enhance extinction recall (indexed by lower SCR) in a dose-response manner; 2) E2 administration will increase vmPFC and decrease dACC and amygdala activations during recall in a dose-response manner. Once the optimal E2 dose has been identified, the R33 phase will examine the impact of E2 administration (relative to Plc) in conjunction with 5 PE sessions in OC users women having significant symptoms of PTSD. Participants will be exposed to the fear conditioning and extinction protocol before and after PE. BOLD signal, SCR as well as symptom severity will be used before and after treatment to test these hypotheses: 1) During extinction recall, both groups will show lower SCR at post- relative to pre-PE, with E2+PE group showing the strongest effect. 2) Extinction-induced activations will be higher in the vmPFC and lower in the dACC and amygdala in post relative to pre-PE, with E2+PE group showing the strongest effect. 3) Information flow between the extinction nodes will improve following therapy (indexed by dynamic-causal modeling), with stronger effects in the E2+PE group. 4) PTSD symptom severity will be lower in the E2+PE group relative to the Plc+PE group following treatment, as well as at the 3 and 6-month follow-up assessments. 5) PTSD symptom reduction will correlate with BOLD and SCR changes observed during extinction recall. These findings will elucidate the neural mechanisms underlying effective exposure treatment for fear-based symptoms, and will reveal how E2 could be an adjunct to enhance the efficacy of extinction-based therapies such as PE.