The individual traceability of cattle in Uruguay has become a reality in recent years. The technological element for the animal is a radio frequency identification (RFID) passive (without battery) ear Tag, which consists on an integrated circuit and a coil (antenna) encased in a plastic package. The identification process is done by a reader device that sends the energy to the Tag through an inductive coupling with the coil/antenna at a certain frequency (134.2 kHz according to ISO 11784/11785 standards). Once the Tag is energized, it responds with its unique code through the same inductive link to the reader. Tags for animal identification are inexpensive, and can be read from distances of approximately 30 cm with a regular portable reader. The limit on the distance is mainly due to the fact that the energy for the Tag to operate is provided by the reader itself, which must establish a signal of at least a few volts of amplitude at the antenna to guarantee the proper operation. Since the near electromagnetic field decays with the cube of the distance, the energy transfer process rapidly becomes unpractical. This is the main issue in terms of flexibility and convenience in the use of these devices. This project focuses on the study of animal identification devices in an active form: it includes the proof of concept, definition of strategy and implementation of integrated circuit blocks oriented to the design of Tags similar to those regularly used in our country, but incorporating a low-cost battery as the power supply which enables the readout from larger distances. The challenge is to achieve the largest possible reading distance with a very low current consumption to extend the Tag's lifetime to several years. In a first approach, a prototype of an active temperature data logger was designed with discrete of-the-shelve components. It was able to measure the temperature and transmit the data to a reader through the same communication protocol of 134.2 kHz RFID Tags. The reception was implemented just demodulating the induced signal in the coil/antenna with a Schottky diode and an RC circuit. Similar reading distances to the ones obtained with a regular passive Tag were achieved. The main limitation was given by the reception circuit, which was unable to detect the presence of the reader when the amplitude of the induced signal at the antenna was below the diode‟s voltage drop, as the distance between the devices increased. On the other hand, the response from the prototype was properly received by the reader at a distance of 45-50 centimeters when the logger was set to continuously transmit. This is the main reason for the later design of an integrated circuit with a more efficient reception circuit, capable of detecting signals of a few millivolts or microvolts. An integrated circuit including the main analog circuit blocks for an active RFID Tag was designed. It consists of a band-pass, high-gain amplifier for the induced signal at the antenna, a detection circuit for the amplified signal and a simple transmission circuit to be driven by a microcontroller. These blocks in conjunction with the antenna and a few discrete capacitors and resistors conformed the prototype of a RFID Tag. This work focused in the design of the amplification and reception circuits, from which it was able to detect the presence of the reader from approximately 4 meters of distance, with a current consumption below 5 µA, to extend the life of a 250 mAh battery for more than 5 years. Transmission distances were not larger than the ones obtained with the discrete prototype. A deep study of the transmission circuit is necessary to further increase this distance; however, this study is not within the scope of this project. The obtained results motivate further research in the field of active RFID identification under the ISO standards.