Extracellular signal-regulated kinases, protein kinase B/Akt and cyclase response element-binding protein play important roles in drug-induced neuroadaptations. Acute psychostimulant exposure rapidly alters the phosphorylation of these proteins in the striatum but less is known about their responses to repeated stimulant administration. In this study the phosphorylated state of these proteins in rat striatum was analyzed by immunoblotting 15 min and 2 h after amphetamine (AMPH)-induced behavioral sensitization. Two weeks after the last dose of 5 mg/kg, i.p. AMPH once daily for 5 days, rats were challenged with 1 mg/kg, i.p. AMPH or saline and sacrificed 15 min or 2 h later. Sensitization to AMPH-induced behavioral activity was observed in AMPH pre-treated rats after AMPH on the challenge day. Phosphorylation of all three proteins was significantly greater 15 min after AMPH in AMPH-pre-treated than in saline-pre-treated rats. Two hours after AMPH challenge in AMPH-pre-treated rats, phospho-extracellular signal-regulated kinase and phospho-cAMP response element-binding protein immunoreactivity was still significantly elevated but not after AMPH injection in saline-pre-treated rats. In contrast, phospho-Akt was down-regulated to the same extent 2 h after acute AMPH or repeated AMPH with an AMPH challenge. These data implicate differential regulation of phospho-extracellular signal-regulated kinase, phospho-cAMP response element-binding protein versus phospho-Akt in sensitized responses to AMPH.