ATPASE activities during development have been studied in rat brain (SAMSON and QuI", 1967), in nerve-ending particle fractions from rat brain (ABDEL-LATIF, BRODY and RAMAHI, 1967) and in the cerebral cortex of the kitten (HUTTENLOCHER and RAWSON, 1968), as well as in preparations of synaptosomes and microsomes of rat brain (ABDEL-LATIF, SMITH and ELLINGTON, 1970). In a previous study, we compa...