Age-related human neurodegenerative diseases are a major social and medical problem. It is therefore logical to take into consideration every theory with an overall approach to neurodegenerative diseases. This environmental proposal relies mainly on data concerning the Western Pacific amyotrophic lateral sclerosis-Parkinsonism-dementia complex (WP ALS-PD) considered as 'a prototypal human neurodegenerative disease' and on extrapolation from it to the bulk of neurodegenerative diseases (NDD). NDD would be due to an accelerated ageing process in certain populations of neurons due to the noxious synergy of (1) increased environmental slow deleterious factors (such as slow toxins) and of (2) decreased environmental protective factors (Mg deficient intake particularly). First, it was observed that three apparently dissimilar conditions occurred at extraordinary high rates in the Guam area: motoneuron disease (ALS), Parkinson's disease (P) and Alzheimer's-like dementia (D). Next, several other foci of endemic ALS-PD were found in Asia and Oceania in three Western Pacific population groups. These included the Chamorro people in Mariana Islands (Guam and Rota), the Auyu and Jakai people of West New Guinea and the Japanese residents of the Kii peninsula (Honshu island). The post-Second World War decline of the occurrence of WP ALS-PD in all three high incidence disease foci coupled with the absence of demonstrable heritable or transmissible factors had led to focus the search for the cause of this degenerative disease on nontransmissible environmental factors that are disappearing as the susceptible population groups acculturate to modern way. Epidemiologic study has shown that preference for traditional Chamorro food is the only one of 23 tested variables significantly associated with an increased risk for PD. An early suggestion incriminated the toxic seed of the false sago palm (Cycas circinalis L) which was used in traditional food and medicine. Laboratory investigation of cycad seed revealed the presence of various toxins and particularly of an 'unusual' non protein aminoacid: L-BMAA (beta-N-methylamino-L-alanine), an excitotoxic aminoacid. This slow toxin presents some structural similarity to another 'unusual' excitotoxic aminoacid: L-BOAA (beta-N-oxalyl-amino-L-alanine), an exogenous neurotoxin present in the grass pea (Lathyrus sativus) whose excessive consumption may cause lathyrism. The excitotoxicity of both L-BMAA and L-BOAA mainly concerns non-NMDA receptors. The neurotoxicity of these aminoacids varies with experimental models failing to induce an experimental model akin to WP ALS-PD or displaying many of the motor-system and behavioral changes of WP ALS-PD. It may be due to the presence of physiological levels of bicarbonate or of various toxic cofactors: bio-organic such as cycasin or inorganic such as pollutant metals e.g. aluminum or manganese, together with the lack of protective factors (e.g. calcium and magnesium deficiencies). Combined Al intoxication with Ca-Mg deficiencies is a reasonable model to investigate the pathogenesis of neurodegenerative diseases and eventually to screen their treatments. It may also be considered as a model of magnesium deficit, but it does not concern simple magnesium deficiency reversible with mere oral physiological magnesium supplementation. Magnesium deficiency cannot result in neurodegenerative disease. Combined Al intoxication with Ca-Mg deficiencies is not reversible through physiological oral magnesium supplementation. It therefore constitutes a type of experimental magnesium depletion model, instrumental in the investigation of the pathogenesis of magnesium depletion and in the screening of its still unknown possible treatments. (ABSTRACT TRUNCATED)