Susceptibility to enoxacin (80 mg/kg, p.o.)-induced convulsions was examined in mice coadministered with fenbufen (100 mg/kg, p.o.) over 24 hr at 3-hr intervals (light 7:00-19:00 hr). There was a marked circadian variation in the incidence of clonic and tonic convulsions and mortality. The susceptibility to enoxacin was higher around 15:00-18:00 hr and lower around 3:00-9:00 hr; the 50% clonic convulsive dose (CD₅₀) at 9:00 and 15:00 hr was 95.0 and 56.5 mg/kg, respectively, its ratio being 1.64. Under these conditions, brain enoxacin level at 15:00 hr was increased 2.43-fold over that at 9:00 hr 30 min after enoxacin administration. Thus, the change of brain enoxacin may contribute to one of the causes of the above circadian variation.