The preoptic hypothalamus has been consistently implicated in the homeostatic regulation of sleep and waking. Recently, it was shown that periods of either spontaneous or forced wakefulness result in the induction in this region of the immediate-early gene c-fos. In this study, antisense oligonucleotides complementary to c-fos mRNA were employed to interfere with the expression of Fos protein. Injections in the rat medial preoptic area of c-fos antisense, but not of sense, oligonucleotides blocked the expression of Fos protein detected immunocytochemically. Rats receiving bilateral antisense injections showed a higher percentage of wakefulness the day after the injection than controls receiving sense or sham injections or antisense injections outside the preoptic area. These results suggest that blocking the expression of Fos protein in the preoptic hypothalamus may interfere with the homeostatic regulation of sleep and wakefulness.