In mice the ED50 of phenobarbital in the maximal electro-shock test was determined for time intervals of 1 to 24 h after oral application. When the concentration of phenobarbital in serum and brain was determined after the respective ED50, it appeared that the phenobarbital concentrations necessary for protection were considerably lower after time intervals of 12–24 h than after 1–8 h. In experiments designed to find a biochemical explanation for this observation, the turnover of noradrenaline in brain was found low after short time intervals, but had normalized after 20 and 24 h. The turnover of 5-HT behaved just opposite: it was enhanced after 2 and 4 h and considerably lowered after the longer time intervals. Thus, the observed difference in central sensitivity to the anticonvulsant effect of phenobarbital may be caused by an imbalance between these two amines in the brain.