Despite numerous experiments showing that administration of neuropeptide Y (NPY) to rodents stimulates feeding and obesity, whereas acute interference with NPY signaling disrupts feeding and promotes weight loss, NPY-null mice have essentially normal body weight regulation. These conflicting observations suggest that chronic lack of NPY during development may lead to compensatory changes that normalize regulation of food intake and energy expenditure in the absence of NPY. To test this idea, we used gene targeting to introduce a doxycycline (Dox)-regulated cassette into the Npy locus, such that NPY would be expressed until the mice were given Dox, which blocks transcription. Compared with wild-type mice, adult mice bearing this construct expressed approximately 4-fold more Npy mRNA, which fell to approximately 20% of control values within 3 days after treatment with Dox. NPY protein also fell approximately 20-fold, but the half-life of approximately 5 days was surprisingly long. The biological effectiveness of these manipulations was demonstrated by showing that overexpression of NPY protected against kainate-induced seizures. Mice chronically overexpressing NPY had normal body weight, and administration of Dox to these mice did not suppress feeding. Furthermore, the refeeding response of these mice after a fast was normal. We conclude that, if there is compensation for changes in NPY levels, then it occurs within the time it takes for Dox treatment to deplete NPY levels. These observations suggest that pharmacological inhibition of NPY signaling is unlikely to have long-lasting effects on body weight.