The choroid plexus was the subject of detailed histochemical and biochemical investigations for long time. However, studies concerned with quantitative analysis of enzymes of energy metabolism and cerebrospinal fluid secretion were the minority. The choroid plexuses of the lateral, third and fourth ventricles of 40 adult rabbits were studied morphologically, histochemically and biochemically for qualitative and quantitative assessment of enzyme activity. Standard histological methods showed no distinguishable differences between the variously located plexuses. Histochemical preparations for esterases, succinate dehydrogenase and carbonic anhydrase gave higher enzymatic reactivity in the fourth ventricle plexus compared to those of the lateral and third ventricles. With all these histochemical methods, ependymal cells showed weaker enzyme reactivity compared to the choroidal epithelium. Microspectrophotometry of the final reaction product of esterases revealed a significant difference in favor of the fourth ventricle plexus against the lateral and third ventricles plexuses (51.6 mM versus 44.6 mM). Similarly, the optical density of succinate dehydrogenase final reaction product was significantly higher in the former plexus than in the latter (0.627 ± 0.0829 versus 0.504 ± 0.0927). Variations in the reaction intensity were seen and groups of non-reactive cells were observed in the variously located choroid plexuses. The origin and identity of these non-reactive cells suggested a population of reserve pial cells, pericytes, or resting choroidal epithelium. Biochemical measurements of carbonic anhydrase specific activity gave significantly higher figures in the fourth ventricle plexus compared to those of the lateral and third ventricles (18.09 ± 1.97 versus 10.05 ± 1.12 ApH unit.min"I.mg"1 soluble proteins); however, the total enzyme activity was in balance on either end of the cerebral aqueduct. The choroid plexuses of various cerebral ventricles showed regional differences that reflected a range of functional levels. These differences, yet, did not reach a point of segregation into three biologically distinct groups.