Ultrasound-modulated optical tomography (UOT) is based on "tagging" light in turbid media with focused ultrasound. In comparison to diffuse optical imaging, UOT can potentially offer a better spatial resolution. The existing Monte Carlo (MC) model for simulating ultrasound-modulated light is central processing unit (CPU) based and has been employed in several UOT related studies. We reimplemented the MC model with a graphics processing unit [(GPU), Nvidia GeForce 9800] that can execute the algorithm up to 125 times faster than its CPU (Intel Core Quad) counterpart for a particular set of optical and acoustic parameters. We also show that the incorporation of ultrasound propagation in photon migration modeling increases the computational time considerably, by a factor of at least 6, in one case, even with a GPU. With slight adjustment to the code, MC simulations were also performed to demonstrate the effect of ultrasonic modulation on the speckle pattern generated by the light model (available as animation). This was computed in 4 s with our GPU implementation as compared to 290 s using the CPU.