1. Brain guanidino compound levels in twelve strains of mice (including El and CBA, both of which are characterized by epileptic dispositions) were analysed fluorometrically after reaction with phenanthrenequinone in alkali solution, using a guanidino compound analyser (JASCO G-520). 2. Taurocyamine, guanidinoacetic acid, N-acetylarginine, creatinine, arginine and methylguanidine in brain were identified and calculated by comparison with the peaks of each authentic sample. Thee analysis sensitivity was very high and less than 0.1 nmole of guanidino compound was sufficient for determination. 3. The concentrations of guanidino compounds in brain varied considerably from strain to strain. Higher taurocyamine levels were found in CBA, El, ddY, AKR, C(58) and RF strains. The highest level was found in the CBA strain, although this variation was not specific for the strain with an epileptic disposition. 4. Higher methylguanidine levels were found in CBA, El, DBA, D(103), C(58) and C(3)H strains. Extraordinary high levels were observed in CBA and El strains. 5. Higher levels of N-acetylarginine were found in DBA, D(103) and C(58) strains. The lowest level was found in the Zb strain. 6. The highest guanidinoacetic acid levels were found in the CBA and El strains. 7. Taurocyamine, guanidinoacetic acid and methylguanidine may play some important role in the seizure mechanism, because their brain levels were high in the strains with an epileptic disposition (CBA and El).