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Rats were kept from birth in light cycles and in light—proof rooms. Biochemical analysis of ATPase activity of the whole cyes on days 10, 25 and 40 postnatally revealed firstly a steady rise with age in Mg—ATPase and a sharp three fold increase between days 10 to 25 in Na—K—ATPase; little Na—K—ATPase activity was observed at day 10. Secondly, light deprivation partially but significantly inhibited the changes in the activity of both enzymes on day 25. The suppression of Na—K—ATPase, however, continued up to day 40. Results from a group transferred from dark to light cycles on day 25 showed the stimulatory effect of light on enzymatic growth of the eye and reversibility of the retarded effects.