Repeated administration of amphetamine results in the well known phenomenon of reverse tolerance or sensitization. However, little is known about cellular and molecular mechanisms underlying acute versus chronic response to amphetamine. In this paper, we investigated the effects of acute (1.5 or 5 mg/kg) and chronic (5 mg/kg/day for 14 days) amphetamine treatment on locomotor activity, stereotypy, Fos immunoreactivity and messenger RNA levels of molecules implicated in dopamine transmission in the rat striatum and substantia nigra. In agreement with other studies, acute amphetamine induced a dose dependent increase in locomotor activity and stereotypy. Also, a comparison between the behavior observed after the first injection and the last injection of amphetamine in chronically treated rats showed sensitization as demonstrated by a higher rating of stereotypy. We have found that acute and chronic amphetamine treatments differently modulate the activity of several output neurons. A double labeling procedure with Fos immunohistochemistry coupled with in situ hybridization demonstrated that acute amphetamine treatment induces Fos immunoreactivity predominantly in striatal neurons expressing substance P messenger RNA (77.07 +/- 1.42%). Only 32.6 +/- 2.07% of Fos immunoreactive neurons expressed preproenkephalin A messenger RNA. In chronic amphetamine treated rats, 56.21 +/- 1.32% of the Fos immunoreactive neurons expressed substance P messenger RNA while 52.12 +/- 1.84% expressed preproenkephalin A messenger RNA. Statistical analysis revealed that this difference is mainly due to a decrease in the density of substance P immunoreactive neurons in chronically treated rats in comparison to acute. Amphetamine treatments induced Fos immunoreactivity in the substantia nigra in non-dopamine neurons. As measured by quantitative in situ hybridization, acute amphetamine induced an increase in substance P, preproenkephalin A and dynorphin messenger RNA levels (+23 +/- 0.05%, +45 +/- 0.07% and +24 +/- 0.05%, respectively). No difference in these increases was observed in relation with the dose injected (1.5 or 5 mg/kg). Chronic amphetamine treatment enhanced only substance P and dynorphin messenger RNA levels (+23 +/- 0.04% and +42 +/- 0.04%, respectively). Neither acute nor chronic amphetamine treatment had any effects on D1 or D2 dopamine receptor messenger RNA levels. Our main conclusions are: (1) in acutely treated rats Fos is essentially expressed by substance P neurons; (2) in chronically treated rats, Fos immunoreactivity is expressed by the two efferent striatal populations (i.e. preproenkephalin A and substance P neurons) and the number of Fos immunoreactive neurons is reduced as compared with acute; (3) neuropeptide messenger RNA levels, but not dopamine receptor messenger RNAs, are affected in the response to acute or chronic treatment with amphetamine.