Rab3a is a small GTP binding protein associated with presynaptic and secretory vesicles that is thought to regulate their targeting to active zones. However, the results in chromaffin cells were contradictory, resulting in facilitation or, alternatively, inhibition of exocytosis. In a previous presentation, we showed by electrochemical techniques that N-terminal fragment of Rab3a contributes to the opening of the early fusion pore. To analyze the effect of Rab3A on vesicle fusion with a technique with different potentialities, we started to perform TIRFM in primary cultures of murine chromaffin cells expressing the pH-sensitive red fluorescent protein pHmScarlet. As the first step, we study the population of events in control conditions. We found two types of events: one type shows a fast increase and a slow decay kinetics in the order of few hundred milliseconds time constant, and the other show also a fast increase but it decays with an atypical fast time constant of approximately 40 ms. Some of these events also show a prolonged dwell time, which can be representative of a sustained fusion pore. The next step will be to analyze the effects of the alternative expression of wild type Rab3a, constitutively activated Q81L and the chimerical Rab3a-22a protein on these populations of fusion events.