Failure of nine-month phenobarbital administration to reverse amygdaloid-kindled seizure susceptibility in cats. Upon completion of left amygdaloid kindling, 4 cats underwent long-term phenobarbital administration during the subsequent 5- to 9-month rest period. Plasma phenobarbital levels were maintained above 15 to 20 micrograms/ml and were restimulated following plasma phenobarbital washout. Three cats served as nonmedicated controls. All 7 cats were subjected to repeated 6-hour sleep monitoring for observation of interictal discharges, which were observed most often in the immediate postictal period. Their frequency decreased gradually throughout the experiment in both the medicated and control animals, but they never completely disappeared except from the contralateral amygdala in 1 medicated animal. Upon primary site restimulation, all of the medicated animals responded with a generalized convulsion once the afterdischarge was induced. When these animals underwent secondary-site amygdaloid kindling, 3 showed a positive transfer effect. The findings suggest that although phenobarbital is a potent anticonvulsant, it has little effect on the acquired seizure susceptibility of previously amygdaloid kindled cats. 