The influence of ageing on the metabolic profile of lateral ventricle choroid plexus epithelial cells from young (3-month-old) and aged (26-month-old) male Wistar rats was studied using enzyme histochemical techniques. The following enzymatic activities related to energy transduction were examined: lactate-(LDH) and succinate- (SDH) dehydrogenases; NADH2-tetrazolium reductase (NADHD) and alpha-glycerophosphate-dehydrogenase (GPDH). The intensity of enzymatic staining within single choroid plexus epithelial cells from young and old animals was assessed microphotometrically. In the choroid plexus epithelial cells of young rats NADHD was the enzymatic activity more heavily stained; cell levels of LHD and GPDH were approximately the same and SDH reactivity was less intense. In old age LDH was reduced by 9.3%, SDH was reduced by 26.1%, NADHD was reduced by 8.6% and GPDH was reduced by 3.6%. The possibility that impaired energy transduction mechanisms at the level of choroid plexus epithelium in old age may influence functional activity of the choroid plexus is discussed.