Eye lens dry weights are commonly used for estimating the age of animals in the wild, but reported relationships are variable. The purpose of this study was to determine why different relationships have been reported using data available for the same species of rabbits.Published results that relate lens weight to age for wild European rabbits from four locations in Australia and cottontail rabbits from two locations in the United States have been reexamined. In addition, the effects of variations in lens preparation have been tested with fresh eyes.It was found that, in previous studies, the logistic type relationship between lens weight and age was interpreted inappropriately through the use of age constants, which imply that lens growth commences before conception. Using gestational time as the constant yields a single formula for each species, and this is consistent with most of the published data. Variations in fixation and drying conditions may be responsible for small differences between different populations.When using lens dry weights as a measure of age, it is recommended that eyes be fixed for at least four weeks and the fixed lenses be dried for two weeks at 85 degrees C or, preferably, three days at 100 degrees C. Any formula, relating age and lens dry weight for any species, must take into account the fact that lens growth commences during gestation not before or after.