We report that FG-7142 (20 mg/kg) differentially increased c-fos in the prefrontal cortex, nucleus accumbens, and striatum of rats 10, 18, 45, and 100 days of age. FG-7142 selectively activated the cortex in adults (70.7+/-3.0%), but the pattern was stronger in nucleus accumbens (83.4+/-9.8%) in younger subjects. These results are consistent with the delayed maturation of the cortex, and show that stress produces more diffuse effects early in life.