In vitro evidence suggests that extracellular signal-regulated protein kinases (ERKs) and Akt (also referred to as protein kinase B) are among the myriad of intracellular signaling molecules regulated by opioid receptors. The present study examined the regulation of ERK and Akt activation in the nucleus accumbens and caudate putamen following acute and chronic morphine administration in the rat. ERK and Akt are activated by phosphorylation, hence the levels of phosphorylated ERK (pERK) and Akt (pAkt) as well as total levels of ERK and Akt protein were measured by Western blot analysis. Male Sprague-Dawley rats received either a single injection of morphine or twice daily injections of morphine for 6 or 10 days. Following acute morphine, pERK levels were significantly decreased in the nucleus accumbens but not in the caudate putamen. Phosphorylated Akt levels in the nucleus accumbens were significantly increased after a single morphine injection. Naltrexone pretreatment prevented both the morphine-induced pERK down-regulation and pAkt up-regulation. Although reductions in pERK levels were evident after 6 days of morphine administration, no differences were observed in pERK levels after 10 days. In contrast to the up-regulation seen after acute morphine, pAkt levels in the nucleus accumbens were significantly decreased after chronic morphine administration. Thus, the differential activation patterns of both ERK and Akt after acute and chronic morphine administration could have important implications for understanding additional pathways mediating opioid signaling in vivo.