An easy method for collecting cerebrospinal fluid (CSF) was developed using a catheter inserted into the cisterna magna of rabbits. Levels of guanidino compounds in CSF collected by this method were measured. Levels of guanidinoacetic acid and creatinine increased at the onset of a pentylenetetrazol (PTZ)-induced convulsion. These levels normalized 2 h after the convulsion. The arginine level started to decrease 2 h after the convulsion, continued to decrease gradually and normalized 4 days after the convulsion. These results suggest that guanidinoacetic acid and creatinine are related to the initiation of PTZ-induced convulsions, and that 4 days are required for the metabolism of guanidino compounds to return to normal after a convulsion in rabbits.