Female rats were placed into complex (EC) or isolated (IC) environments for 45 days, beginning at 450 days of age. Golgi-Cox stained layer IV stellate and layer III occipital cortical pyramidal neurons were analyzed for numbers and lengths of dendritic branches and the amount and location of dendritic material with respect to the soma. In both cell populations, neurons from EC rats had significantly more dendritic material, with differences occurring throughout the dendritic field. In the stellate population, EC neurons had significantly longer terminating (non-bifurcating) branches, and more second and fifth order branches. In pyramidal cells, EC neurons had significantly more branches at each of orders 2-5 as well as significantly longer terminating branches. This report is the first to describe substantial dendritic alterations subsequent to differential housing in middle-aged rats. These results, taken with previous work, suggest that experience may affect neuronal structure over much of an animal's lifespan.