Naturally occurring estrogen-like molecules in plants (phytoestrogens), present via soy, in animal diets are known to alter brain morphology and reproductive endocrine parameters. This study characterized dietary phytoestrogens' effect on retinal thickness in male and female Long-Evans rats. Experiment 1a and 1b: upon arrival (50 day-old) animals received either a phytoestrogen-rich diet (containing 600 microg phytoestrogen/g diet; referred to as Phyto-600) or a diet low in phytoestrogens (Phyto-free). Males remained on these diets until 140 days of age (females until 160 days of age). In both sexes a significant (but opposite) diet difference in retinal thickness was identified. Male Phyto-600 and female Phyto-free animals had significantly greater retinal thickness compared to Phyto-free males and Phyto-600 females, respectively. Experiment 2: male or female rats were raised from conception on either the Phyto-600 or Phyto-free diet until sacrifice at 75 days of age. Consistent with experiment 1, males exposed to the Phyto-600 diet lifelong had significantly greater retinal thickness than lifelong fed Phyto-free males (no significant differences were identified in females). These data suggest that phytoestrogens influence rat retinal characteristics in a sexually dimorphic manner (more robust effect in males vs. females) and that this influence can occur even in adulthood.