Ma-huang is a traditional Chinese medicinal herb derived from EPHEDRA: sinica Stapf and other EPHEDRA: species, used to treat asthma, nose and lung congestion, and fever with anhidrosis. It contains 0.5-2.5% by weight of total alkaloids, of which ephedrine accounts for 30 to 90%. Recently, large amounts of ma-huang were used as a source of ephedrine in many dietary supplements formulated for weight reduction, because ephedrine has been found effective in inducing weight loss in diet-restricted obese patients. However, indiscriminate consumption of ma-huang-containing products has resulted in many cases of poisoning, some of which were fatal. The objective of this study is to investigate the relative toxicity of ma-huang extracted under different conditions. The toxicities of various extracts were assayed using MTT colorimetry on a battery of cell lines, while ephedrine alkaloids were analyzed with HPLC. The results are summarized as follows. (1) The cytotoxicity of all ma-huang extracts could not be totally accounted for by their ephedrine contents, suggesting the presence of other toxins in the extracts. (2) Grinding was a significant condition enhancing the toxicity of the extracts. (3) The relatively high sensitivity of the Neuro-2a cell line to the toxicity of ma-huang extracts suggests that the toxic principles were acting on neuronal cells. (4) One condition to produce a ma-huang extract with high ephedrine-to-toxins ratio would be to boil the whole herb for two h.