We have demonstrated a prototype ZnO nanowire based nanogenerator (NG) that can effectively generate electricity inside biofluid when stimulated by ultrasonic waves. The potential of increasing output current and voltage was illustrated by connecting multiple NGs in parallel and serial, respectively, clearly demonstrating the possibility of raising output power by three-dimensional integration and architecture. The output current was increased by 20-30 times and reached as high as 35 nA when a 2 mm2 size NG was placed at a region where the ultrasonic waves were focused. This work unambiguously shows the feasibility of NGs for power conversion inside biofluid. It sets a solid foundation for self-powering implantable and wireless nanodevices and nanosystems in biofluid and any other type of liquid.