The aim of this study is to evaluate the effects of pineal gland functional state on the prooxydant processes and antioxidant system in the gingival tissues. Male rats were assigned into one of the following groups in accordance with the duration of photoperiod: 1) control--natural daylight; 2) permanent darkness for 14 days; 3) permanent light for 14 days. The following parameters were measured in gingival tissues and the blood serum: 1) prooxidant factors (dieneconjugates--DC and malonic dialdehyde--MD); 2) antioxidant enzymes (superoxide dismutase--SOD and catalase). The present findings indicate that the gingival tissue of rats reacts to the changes in the duration of photoperiod by peroxidation and activity of antioxidant enzymes. The antioxidant-prooxidant index under dark conditions (high function of the pineal gland) was lower than under condition of permanent light ("physiological" pinealectomy). Different durations of photoperiod change the intensity of free radical oxidation and the activity of antioxidant enzymes at the systemic level (blood serum) and much more at the organ level (tissues of gingiva). Our data suggest that the gingival tissues possess rather powerful protective antioxidant system, which depends on the functional state of the pineal gland.