1 Introduction.- 2 Materials and Methods.- 2.1. Animals.- 2.2 Electron Microscopy.- 2.3 Immunocytochemical Experiments.- 2.4 In Situ Hybridization Histochemistry.- 2.5. Peripheral Nerve Axotomy.- 2.6 Data and Image Analysis.- 3 Results.- 3.1 Normal Morphology of the Mesencephalic Trigeminal Nucleus.- 3.1.1 Ultrastructure of the Mesencephalic Trigeminal Nucleus.- 3.1.2 Synaptic Organization of the Mesencephalic Trigeminal Nucleus.- 3.2 Neurochemistry of the Mesencephalic Trigeminal Nucleus.- 3.2.1 Classical Neurotransmitters.- 3.2.1.1 Excitatory Amino Acids.- 3.2.1.2 Inhibitory Amino Acids.- 3.2.1.3 Monoamines.- 3.2.2 Neuropeptides.- 3.2.2.1 Substance P.- 3.2.2.2 Bombesin.- 3.2.2.3 Enkephalins.- 3.2.2.4 Cholecystokinin.- 3.2.2.5 Vasoactive Intestinal Polypeptide.- 3.2.2.6 Calcitonin Gene-Related Peptide.- 3.2.2.7 Galanin.- 3.2.2.8 Neuropeptide Y.- 3.2.2.9 Arginine Vasopressin.- 3.2.3 Other Neuroactive Agents.- 3.2.3.1 Nitric Oxide.- 3.2.3.2 Calcium-Binding Proteins.- 3.2.4 Receptors in Mesencephalic Trigeminal Nucleus Neurons.- 3.2.5 Axotomy-Induced Changes in Neurochemical Content of Mesencephalic Trigeminal Nucleus Neurons.- 4 Discussion.- 4.1 Structural and Synaptic Organization of the Mesencephalic Trigeminal Nucleus.- 4.2 Connections of the Mesencephalic Trigeminal Nucleus.- 4.3 Neurochemistry of the Mesencephalic Trigeminal Nucleus.- 4.3.1 Classical Neurotransmitters and Their Known Receptors.- 4.3.2 Neuropeptides.- 4.3.3 Other Neuroactive Substances.- 4.4 Neurochemical Plasticity of Mesencephalic Trigeminal Nucleus Neurons.- 4.5 Functional Considerations.- 4.6 Conclusions and Future Directions.- 5 Summary.- References.