Breakdown pressure of induced hydraulic fractures from circular boreholes is investigated experimentally utilizing a novel, two-dimensional (2-D) fracturing cell. The objective of this study is to investigate how breakdown pressure in fracturing experiments is affected by borehole notch properties that include notch permeability, notch length and its orientation relative to the applied far-field stresses. We show that test specimens with an initial borehole notch have markedly lower breakdown pressures compared to boreholes with no notches. We demonstrate that if the borehole notch is made impermeable to the injection fluid by plugging the notch with a flexible adhesive, the breakdown pressure of the specimen with an impermeable notch is virtually the same as a specimen without an initial notch. Also, the breakdown pressure decreases as the borehole notch becomes longer and increases as the notch deviates from the direction of the applied maximum far-field stress. The measured breakdown pressure values obtained in our laboratory experiments are compared with theoretical predictions from analytical solutions available in the literature for the effect of notch length and orientation.