Proteins can be key biochemical markers for evaluating the functional activity of nervous system cells. They are involved in the proliferation and differentiation of nerve and glial cells and the arrangement of many metabolic functions of the brain. This study aimed to analyze the concentration of proteins in the neurons of the lateral preoptic nucleus (LPON) of the hypothalamus in mature and old rats under standard and altered lighting conditions. Our results show that the concentration of proteins in mature rats was significantly higher than in old rats (0.274±0.0017 optical density units), with a predominance of carboxyl groups, indicating a high intensity of protein metabolism. Additionally, we found that changes in the lighting regime have a differential effect on the optical density of specific staining for protein in LPON neurons. Specifically, light deprivation did not significantly alter the optical density of specific staining for protein in neurons of the hypothalamus LPON of mature rats regardless of the period of the day, while in old rats, the intensity of protein staining decreased. Light exposure, on the other hand, led to an increase in the average color intensity for protein in neurons of the hypothalamus LPON in mature rats (0.326±0.0014 optical density units), while in old rats, a decrease in the average color intensity for protein in neurons of the hypothalamus LPON was observed (0.196±0.0017 optical density units).