Multiplex differential display (MDD), a modification of differential display reverse transcriptase polymerase chain reaction (DD-PCR), was used to identify cocaine-dependent regulation of previously known and unknown gene products. Direct comparison of the MDD amplification profiles of duplicate, total RNA samples from the caudate putamen (CPu) of either vehicle or cocaine treated Sprague-Dawley rats indicated that the relative induction of a 240 bp (8G247) product, likely to represent c-fos mRNA, closely paralleled changes in c-fos mRNA as measured by Northern blot analysis. MDD and Northern blot analysis also revealed substantial repression of another PCR product (8G226) at 1 h and 1 day after repeated administration of cocaine. At 2 days after cocaine exposure, the level of 8G226 had returned to control levels. The DNA sequence of 8G226 exhibited near identity with a mouse zinc-finger protein (PZf) and is thus likely to represent a transcriptional regulator. Interestingly, the repression of 8G226 immediately after cocaine treatment is in direct contrast to the cocaine-dependent increase in expression documented for NGFI-A, another zinc-finger protein which also functions as a transcriptional regulator. Detailed characterization of the prolonged reduction in the expression of 8G226 may lead to the identification of additional regulatory pathways that produce changes in cellular response after repeated cocaine exposure.