1. Brain guanidino compound levels in ddY mice were analysed in the pre-convulsive stage, during convulsion and at the post-convulsive stage of convulsive seizures induced by pentylenetetrazol or electric stimulation. 2. Significantly increased levels of taurocyamine was found in the pre-convulsive stage, just after pentylenetetrazol injection. This recovered temporarily to normal levels during convulsion, though the level increased again in the post-convulsive stage. Marked increases in creatinine were observed throughout the convulsive seizure induced by pentylenetetrazol; i.e. in the pre-convulsive stage, during convulsion and in the post-convulsion stage. 3. Significantly decreased levels of taurocyamine were found during electrically induced convulsions. Increased levels of creatinine were found in the pre-convulsive stage.