Amyotrophic lateral sclerosis: fasting plasma levels of cysteine and inorganic sulfate are normal, as are brain contents of cysteine. Recent reports suggest that amyotrophic lateral sclerosis (ALS) is caused by one or more unidentified neurotoxins that are poorly metabolized in patients to less toxic and more readily excreted compounds, and that a genetically determined defect in cysteine degradation and in inorganic sulfate production is the mechanism underlying a failure to metabolize xenobiotics normally in ALS. We measured concentrations of total cysteine and of inorganic sulfate in the plasma of age-matched groups of ALS patients and healthy control subjects and found no differences. L-Cysteine, a putative endogenous neurotoxin in ALS, was present in equal concentrations in autopsied brain from ALS patients and controls. 