The effect of haloperidol (HAL) on neurotensin (NT) levels in various structures of the rat brain was evaluated using an immunoperoxidase method. Adult, male Sprague-Dawley rats were given intraperitoneal injections of either HAL (2 mg/kg) or vehicle at twenty-four and four hours prior to sacrifice. The brains were fixed, cut at 50 micron on the vibratome, and prepared to demonstrate NT immunoreactivity, or its absence following appropriate control incubations. The distributions of NT-immunoreactive (IR) cell bodies were plotted using the camera lucida, and the numbers of NT-IR neurons in various structures were recorded. The numbers of NT-IR perikarya in striatal and ventral striatal structures of HAL-treated rats greatly exceeded those observed in the same structures of control animals. In other NT-IR rich regions including the bed nucleus of the stria terminalis, central amygdala, hypothalamus and septum, HAL and control values did not differ. Conversely, HAL treatment appeared to effect a decrease in the number of immunoreactive perikarya in the medial amygdala and caudal part of the endopiriform area. It was noted that in brain regions where D-2 receptors are reported to be numerous, the number of NT-stained cells increased following HAL treatment, whereas in regions where D-1 receptors predominate, the number remained stable or decreased. Subjective evaluation of axon terminal immunoreactivity revealed a change only in the globus pallidus, where the proportional area of that structure exhibiting NT-immunoreactivity expanded following HAL.