A laboratory prototype of a time-resolved photoacoustic mammograph, based on a parallel plate geometry is presented. Light is delivered from a Q-switched Nd:YAG laser using fiber-optic bundles which can be mechanically scanned across the surface of a phantom. The ultrasound signals produced by the photoacoustic effect are measured in a transmission mode, using a large-area ultrasound detector matrix. Signals from the matrix are acquired using fast digitizers. Various performance studies of the system are presented. A breast phantom of dimensions (150x120x60)mm was created based on poly(vinyl alcohol) (PVA) gel, which can be imparted with the average optical scattering properties of breast tissue by a simple process of freezing and thawing of an aqueous poly(vinyl alcohol) solution. The acoustic properties are also found to match those of breast tissue. Such a photoacoustic breast phantom was embedded with several tumour-simulating inhomogeneities. These inserts were also based on poly(vinyl alcohol) gels, appropriately dyed at the time of formation, to possess various optical absorption coefficients, between 2 and 7 times that of the background. Using the signals collected from regions-of-interest (ROI) in the volume of the phantom, three-dimensional images were obtained using a modified delay-and-sum beamforming algorithm. The results indicate that photoacoustics, as embodied in this instrument, has a potential for detecting tumours in the breast.