The study was designed to investigate the effect of exposure to long-term constant darkness, starting from the prenatal period or from birth, on the retinol (vitamin A) content in tissues of rats. Females were kept in standard light (LD) or in constant darkness (DD) during pregnancy. The LD females and their offspring after birth were divided into two groups, one of which was left in the same lighting conditions (LD, control), and the other group was switched to the darkness regime (LD/DD). The DD females and their offspring (DD/DD) were kept in the dark. Adults and the young were separated after the suckling period. The retinol content in offspring's tissues (liver, kidneys, heart and skeletal muscle) was determined at the age of 2 weeks, 1, 2, 3, 6, 12, 18 and 24 months by HPLC. Constant darkness had a modulating effect on the age-related dynamics of retinol content in the tissues. The level of the vitamin was higher compared to the control animals in the liver of 2-week-old LD/DD rats and in the kidneys of 1-month-old rats of both experimental groups. The retinol content in the heart of 2-month-old DD/DD rats, on the contrary, was significantly lower than in the control. The retinol level in the liver of 12-month-old LD/DD rats was higher compared to the control animals. The effect of constant darkness on retinol level in tissues depended on the ontogenesis stage at which the experimental exposure began, the tissue type, and the animal's age. The retinol content in the tissues of rats kept in constant darkness indicates metabolic changes that were more pronounced in young animals. K e y w o rd s: vitamin A; constant darkness; light; circadian rhythm; age F o r c i t a t i o n: Ilyina T. N., Baishnikova I. V., Khizhkin E. A. Effect of long-term constant darkness on retinol in peripheral tissues of rats.