Huntingtin-associated protein 1 (HAP1) is a binding partner of the huntingtin protein responsible for Huntington’s disease, with postulated roles in vesicle trafficking and microtubule transport. The current research provides the first physiological evidence supporting a role for HAP1 in regulation of both exocytosis and endocytosis. Amperometry, utilising chromaffin cells from both WT and HAP1 KO mice, was used to investigate the role of HAP1 in exocytosis. There was a significant reduction in the number of exocytosis events seen in cells from HAP1 KO animals, compared with WT cells. Whilst there were no significant changes in spike parameters the pre-spike foot signal was prolonged in HAP1 KO cells, suggesting that HAP1 may play a role in fusion pore stabilisation. A reduction in the size of the readily releasable pool of vesicles was also observed in HAP1 KO cells, consistent with a reduction in the number of morphologically docked vesicles seen in these cells using electron microscopy. The role of HAP1 in endocytosis was also investigated using patch clamp capacitance measurement, with this data showing a reduced rate of membrane retrieval in the HAP1 KO cells compared to WT. Using a proteomics approach we identified several novel HAP1 interactions with proteins involved in exocytosis, endocytosis and protein trafficking and the mislocalisation of one of these proteins in HAP1 KO neurons. These results therefore suggest, for the first time, the involvement of HAP1 in regulation of both exocytosis and endocytosis, as well as providing mechanisms in which this regulation might be accomplished based on newly identified protein interactions.