Nine volatile N-methyl-N-formylhydrazones were identified from fresh false morel mushrooms, Gyromitra esculenta, at an average combined level of 57 mg/kg. Using high-resolution glass capillary GLC, a method was developed for the control of hydrazone residues in false morel products. Investigation was made of the decrease of levels of hydrazones during boiling and drying processes and of the effect of process conditions on the level of toxin residues. After prolonged drying, the level of hydrazone residues fell below 3 mg/kg of dried mushroom, and a boiling time of 10 min was required to reduce the level below 1 mg/kg. The results were compared with measured values of the toxicity of the main component of the residues, acetaldehyde N-methyl-N-formylhydrazone, and with an estimation of the suitability of false morel as an edible mushroom.