The nervous form of magnesium imbalance represents the best documented experimental and clinical aspects of magnesium disorders. The nervous form of primary magnesium deficit (MD) in the adult appears as the best descriptive model for analysis of the symptomatology, aetiology, physiopathology, diagnosis and therapy of the most frequent form of MD. Nervous hyperexcitability due to chronic MD in the adult results in a non-specific clinical pattern with associated central and peripheral neuromuscular symptoms, analogous to the symptomatology previously described in medical literature as latent tetany, hyperventilation syndrome, spasmophilia, chronic fatigue syndrome, neurocirculatory asthenia and idiopathic Barlow's disease. On encountering this non-specific pattern, the signs of neuromuscular hyperexcitability are of much greater importance. Trousseau's sign is less sensitive than Chvostek's sign, but their sensitivities are increased by hyperventilation (Von Bondsdorff's test). Examination of the precordial area will be conducted in order to search clinical stigmata of mitral valve prolapse (MVP) which is a frequent dyskinesia due to chronic MD (about a quarter to one-third of cases). The electromyogram (EMG) shows one (or several) trains of autorhythmic activities beating for more than 2 min of one of the three tetanic activities (uniplets, multiplets or 'complex tonicoclonic tracings') during one of the three facilitation procedures: tourniquet-induced ischaemia lasting 10 min. post-ischaemia lasting 10 min after the removal of the tourniquet and hyperventilation over 5 min. A repetitive EMG constitutes the principal mark of nervous hyperexcitability (NHE) due to MD. The echocardiogram (ECC) is the best tool for detecting MVP, the 2-dimensional ECC with pulsed Doppler being more accurate than time-motion ECC. The routine ionic investigations comprise five static tests: plasma and erythrocyte magnesium, plasma calcium and daily magnesiuria and calciuria. An evaluation of magnesium intake is desirable. Normal concentrations of magnesium in blood do not rule out the diagnosis of the nervous form of primary chronic MD. The histograms of MD group reveal Gaussian type magnesaemias with significantly lower means and the constituent elements can be individually hypo- (one-third of cases), normo- (about two-thirds of cases) and even, exceptionally, hyper-magnesaemic. The diagnosis of MD requires an oral magnesium load test. At physiological dose (5 mg of Mg/kg/day), oral magnesium is totally devoid of the pharmacological effects of parenteral magnesium. Corrections of symptomatology by this oral physiological magnesium load is the best proof that it was due to magnesium deficiency. In particular clinical forms, more sophisticated studies may be useful: standard and quantitative electroencephalograms, electropolygraphic studies of afternoon sleep, electronystagmography, optokinetic test, skin conductance reflex, psychometric inventories, standard or monitoring electrocardiogram, treadmill test, other static and dynamic investigations: e.g. ionized free Mg2+, lymphocyte Mg, brain Mg, cerebrospinal Mg, Mg balance, Mg parenteral load test, glucose load, and even radio-isotope study, the only one able to reveal intestinal magnesium hypersecretion. Nervous primary chronic MD progresses by phases of decompensation against a background of latency. Marginal magnesium deficiency, that is to say an insufficient magnesium intake which merely requires simple oral physiological supplementation, is fundamental in the aetiology of primary magnesium deficit. However a constitutional homeostatic lability of the nervous system or of magnesium metabolism such as belonging to the B35 type of HLA group must be involved. Part of the aetiology of this magnesium deficit is a magnesium depletion, where the disorder which induces magnesium deficit is related to a dysregulation of the control mechanisms of magnesium status which requires a more or less difficult