The toxicity of monomethylhydrazine (MMH) administered intraperitoneally in the cat was studied by reference to behavioral and neurophysiological indices. The acute toxicity LD50 value for MMH was established as 15 mg/kg, and the CD50 as 7 mg/kg. Doses of 18, 9, and 5 mg/kg were then studied systematically in an effort to classify lethal, convulsive and subconvulsive symptoms. For these doses, a preconvulsive syndrome was described involving recurrent and sustained symptoms including vomiting, panting, rapid respiration, viscous salivation, hyperactivity and subcortical seizure activity. The onset latency of these symptoms was directly related to dose. Several lines of evidence suggested at least a partial independence between the biochemical and neurophysiological events responsible, on the one hand, for convulsions, and on the other for this preconvulsive syndrome. Convulsions were specifically delayed or prevented in animals trained to suppress movement through the use of a special EEG conditioning technique.