Rabbit lenses were incubated, in vitro, for 20 hr. During the first 60 min of the incubation period, the lenses were exposed to hydrogen peroxide at specified concentrations. Deranged electrolyte balance was observed in lenses exposed to hydrogen peroxide concentrations exceeding 0.1 mM.Lenticular sodium pump activity, as evidenced by lens86Rb uptake, was reduced to 61% of control when examined immediately after exposure to 0.5 mM hydrogen peroxide for 1 hr. Upon further incubation in peroxide-free medium, the sodium pump activity of hydrogen peroxide treated lenses continued to deteriorate. The deterioration of sodium pump activity in lenses transiently challenged with hydrogen peroxide was not altered by supplementation of the incubation medium with antioxidants or by substitution of calcium in the medium.Lenticular potassium permeability, as evidenced by the rate of86Rb efflux from the lens, was found to be elevated immediately after exposure to 0.5 mM hydrogen peroxide for 1 hr. However, no further change in the rate of86Rb efflux was noted when the hydrogen peroxide treated lens was incubated for an additional 19 hr in peroxide-free medium.Na, K-ATPase and Ca-ATPase activities were measured in membrane preparations isolated from lenses that had been exposed to 0.5 mM hydrogen peroxide and then cultured in control medium for 19 hr. Neither of the transport enzyme activities was diminished in the hydrogen peroxide treated lenses indicating that the deterioration in sodium pump activity in lenses temporarily exposed to hydrogen peroxide is not due to a progressive loss of Na, K-ATPase enzyme.