Neurofibromatosis type I is a common autosomal dominant disease characterized by formation of multiple benign and malignant tumors. People with this disorder also experience chronic pain, which can be disabling. Neurofibrinomin, the protein product of the NF1 gene (neurofibromin gene (human)), is a guanosine triphosphate activating protein for p21^ras. Loss of NF1 results in an increase in activity of the p21^ras transduction cascade. Because of the growing evidence suggesting involvement of downstream components of the p21^ras transduction cascade in the sensitization of nociceptive sensory neurons, we examined the stimulus-evoked release of the neuropeptides, substance P and calcitonin gene-related peptide, from primary sensory neurons of mice with a mutation of the Nf1 gene (neurofibromin gene (mouse)) (Nf1+/−). Measuring immunoreactive substance P and immunoreactive calcitonin gene-related peptide by radioimmunoassay, we demonstrated that capsaicin-stimulated release of neuropeptides is three to five-fold higher in spinal cord slices from Nf1+/− mice than from wildtype mouse tissue. In addition, the potassium and capsaicin-stimulated release of immunoreactive calcitonin gene-related peptide from cultures of sensory neurons isolated from Nf1+/− mice was more than double that from cultures of wildtype neurons. Treatment of wildtype sensory neurons with nerve growth factor for 5–7 days mimicked the enhanced stimulus-evoked release observed from the Nf1+/− neurons. When nerve growth factor was removed 48 h before conducting release experiments, nerve growth factor-induced augmentation of immunoreactive calcitonin gene-related peptide release from Nf1+/− neurons was more pronounced than in Nf1+/− sensory neurons that were treated with nerve growth factor continuously for 5–7 days. Thus, sensory neurons from mice with a heterozygous mutation of the Nf1 gene that is analogous to the human disease neurofibromatosis type I, exhibit increased sensitivity to chemical stimulation. This augmented responsiveness may explain the abnormal pain sensations experienced by people with neurofibromatosis type I and suggests an important role for guanosine triphosphate activating proteins, in the regulation of nociceptive sensory neuron sensitization.