The effects of the indirect dopamine receptor agonist cocaine in the striatum on levels of mRNAs of the immediate-early gene c-fos and the neuropeptides dynorphin, substance P, and enkephalin were analyzed with quantitative in situ hybridization histochemistry. Both single (acute) and repeated (twice a day for 4 d) systemic injections of cocaine (3.75– 30 mg/kg) to rats resulted in dose-dependent, regionally specific elevations of mRNA expression in striatal neurons. A single drug treatment elevated c-fos mRNA expression, whereas repeated treatments resulted in little c-fos expression but elevated dynorphin mRNA levels. Both the regional and temporal patterns of gene expression revealed an inverse relationship between dynorphin and c-fos expression. This relationship was examined in a time course experiment in which cocaine (30 mg/kg) was administered for 1, 2, 3 or 4 d. Basal levels of dynorphin expression were relatively high in the ventral striatum, including the nucleus accumbens, a ventrolateral region, and an area along the medial bank of the striatum. A single injection of cocaine induced c-fos mRNA in striatal areas with low basal expression of dynorphin. Thus, c-fos mRNA induction was highest in the dorsal central striatum, where basal dynorphin mRNA levels were lowest. In this region, dynorphin mRNA expression increased on subsequent treatment days parallel to diminished c-fos mRNA induction. Changes in substance P. mRNA levels appeared to match directly both the temporal and regional patterns of c-fos induction. Enkephalin mRNA expression was altered, but only slightly, by these cocaine treatments. Statistical analysis of the regional patterns of basal and altered mRNA levels shows a unique inverse relationship between basal dynorphin expression and c-fos induction by cocaine. Further evidence of this relationship is provided by the dose-dependent blockade of cocaine-induced c-fos expression by spiradoline, a dynorphin agonist. Together, these results suggest that the restricted regional pattern of cocaine-induced c-fos expression is related, in part, to the basal level of dynorphin expression, and that cocaine treatment elevates dynorphin expression in striatal regions with a strong c-fos response, thereby limiting subsequent c-fos induction by cocaine. These findings lead to the hypothesis that dynorphin acts to regulate the responsiveness of striatal neurons to dopamine stimulation.