A wireless implantable sensor network system (WISNS) is designed for in vivo monitoring physiological signals of a population of animals. WISNS can simultaneously monitor more than 15 animals, communicating three kinds of analog information among sensor nodes. Analog signals are transmitted to relay node at 800-KHz carrier by AM. Relay nodes digitalize and package them into messages, and then forward to the Wireless sensor network by Nordic RF technology (NWSN). Smaller overall dimensions (<;2 cm3), lower power regulation, and dedicated packaging make the system suitable and compatible for implantable devices. The implantable sensor node, protocol stack of NWSN, and performance of the system are evaluated and optimized with ECG monitoring test of rats. Compared with those commercially available sensor nodes, our implantable one is leading in the weight and volume miniaturization, and our WISNS solution shows huge potential in achieving the compatibility of different animals.