Lens myo-inositol (MI) content is regulated by a pump-leak system consisting of an active Na-depen-dent MI transport and its passive permeability through the membrane. We measured the active MI uptake and membrane permeability in lenses of rats maintained on a 50% galactose diet for 1, 3 and 7 days. After only 1 day of galactose feeding, active MI uptake in the lens was reduced dramatically by 74% compared to age-matched control lenses; by day 3, active Ml transport was decreased by 89% and it was undetectable by day 7. The passive membrane permeability was determined by measuring (a) the passive Ml influx and (b) the3H-sorbitol flux. After 1 day of galactose feeding, the membrane permeability increased such that within 3 days it increased to 5–6 fold. Galactose feeding also led to a rapid increase in lens polyol content. After 1 day, lens polyol increased to 53μmol/g wet wt compared to a control value of 0.35μmol/g wet wt and increased further to 65 and 72μmol/g wet wt after 3 and 7 days of galactose feeding respectively. Lens galactose accumulation was low (3μmol/g wet wt) up to 7 days; however, it was rapidly increased after 7 days. Our results indicate that galactose feeding rapidly interfered with MI homeostasis by a severe depression of active MI transport and a rapid increase in membrane permeability. These interferences of MI homeostasis correlate with the appearance of high polyol levels.