The tyrosine content of eye lenses of known-age old-field mice (Peromyscus polionotus) were analyzed colorimetrically. The soluble fraction increases linearly with age from 45 to 308 days, but thereafter is not related to age. The insoluble fraction varies curvilinearly with age to at least 750 days. Accumulation of soluble protein represents growth, whereas conversion of soluble to insoluble protein is regarded as aging. Aging, as measured by the insoluble fraction, is much less variable than growth, and affords an age estimating technique with unprecedented accuracy for small mammals. The 95 per cent confidence limits about a predicted age of 100 days are 96 to 107 days for a mean prediction, 70 to 147 days for individual prediction. Similar values for 400 days are 390 to 426 and 285 to 583 days, respectively.