The effect of tryptophan and three major metabolites of tryptophan on the rabbit lens electrolyte balance was examined. Even at 3×10−3M tryptophan, tryptamine and xanthurenic acid had no effect on lens sodium, potassium and calcium content after a 20 hr incubation. Small but significant changes in lens sodium and potassium content were induced by 20 hr incubation in 2×10−3M 3-hydroxy-DL-kynurenine and 10−3M hydroxyanthranilic acid. Lens Na,K-ATPase activity was unaffected by 3×10−3M tryptophan. Tryptamine reduced, to a small extent, Na,K-ATPase activity at 3×10−3M, but was without effect at 10−3M. 3-hydroxy-DL-kynurenine (2×10−3M) and 3-hydroxyanthranilic acid (10−3M) inhibited Na,K-ATPase activity by 27% and 30% respectively. Only 3-hydroxy-DL-kynurenine had a demonstrable effect on86Rb uptake.It is concluded that, in contrast to findings in the amphibian lens, metabolites of tryptophan have minimal or no detrimental effects upon rabbit lens electrolyte balance under thein vitroconditions of these experiments. However, this does not rule out a long term effect of these compounds should they accumulate in the lens.