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Rabbits' lenses contain about 100 μmol kg
−1 wet wt (145 μmol kg
−1 water) of zinc. This metal appears to be quite uniformly distributed throughout the organ and more than 90% is firmly incorporated into the tissue so as not to be readily exchangeable. The concentration of Zn in the aqueous and vitreous humors is about 10
−5
m (one-fourth the concentration in the blood serum). Lenses incubated in vitro can accumulate Zn from solutions containing this concentration of the metal. This process is concentration-dependent and is increased following damage produced by metabolic inhibitors. The process probably involves diffusion and is increased in the presence of a low external calcium concentration and the Ca ionophore A 23187. Amino acids which are known to bind zinc did not influence its accumulation by the lens, with the exception of cystine which increased it. Accumulated Zn (using
65Zn as a tracer) was able to leave the lens, but this process was quite slow and was reduced by the presence of lanthanum and low Ca concentrations. It is suggested that Ca and Zn may share common binding sites in the tissue, and they could be utilizing the same channels to cross the cell membrane.