Nerve growth factor (NGF) is thought of as a target-derived factor responsible for the survival and maintaining the phenotype of specific sets of peripheral and central neurons during development and maturation. Recently, using physiological techniques, we have shown that specific functional types of nociceptive sensory neurons require NGF, first for survival during development in utero and then for their normal phenotypic development (but not survival) in the early postnatal period. In adulthood, the physiological role of NGF changes dramatically and here it may serve as a link between inflammation and hyperalgesia. Despite apparent changes in NGF's mode of action as the animal matures, it always interacts specifically with nociceptive sensory neurons.