To clarify the quantitative and qualitative changes in type IV phosphodiesterase (PDE IV) with aging, phosphodiesterase (PDE) activity and [3H]rolipram binding in the cytosolic fraction from the brains of young and aged rats were examined. In all areas of the aged (100-week-old) rat brain except for hippocampus, the PDE activity was decreased by about half that in the young (10-week-old) animals. However, inhibition % by 100 microM rolipram and by 100 microM isobutylmethylxanthine (IBMX) was not significantly different between the aged and young rats. On the other hand, [3H]rolipram binding and rolipram-sensitive PDE activity did not change with aging in the hippocampus, although both were decreased in other brain regions. These results suggest that PDE IV does not decrease with aging and maintains its cAMP degrading activity in the hippocampus. It may be involved in the dysfunction of the hippocampus with aging.