Neurotransmitter and hormone receptors serve as biosensors for specific chemical signals ranging from low-mol-wt compounds to complex polypeptides. On binding of the target transmitter or hormone, signal amplification and transduction in biologic systems occurs via a variety of mechanisms, ranging from depolarization of neural membrane, G protein-linked synthesis of second messengers, to activation or inhibition of expression of target genes. The combination of these sensitive and specific sensing receptor proteins with electrochemical, optical, and acoustic technologies to form analytical devices is an attractive concept. These receptor-based biosensors could potentially find applications in the medical, diagnostics, food, military, and environmental areas.