PetSafe UltraLight Receiver is the world's smallest one weighing only 1.5 ounces and measures 2 1/8" x 3/4" x 3/4". It uses two 6-volt RFA-67D-11 batteries (included) with an average battery life of 7 months. It is recommended for dogs or cats from 8 to 100 lbs. that are easy or average to train. For use with the PRF-3004W or PRF-304W.