The seasonal influence on circadian oscillations of serum thyroid hormones has been confirmed in the laboratory rat, an animal exhibiting low photoperiodic activity. The aim of this paper was to study the influence of various photoperiods, applied in a single season, on circadian variations in the levels of thyroid hormones in male Wistar rats. After 6-weeks of adaptation to artificial light-dark regimens (LD) 08:16 h, 12:12 h, 16:08 h, and to the standard housing conditions, the rats were examined in 3 h intervals in the course of 24 h in December. The concentrations of thyroxine (T4), triiodothyronine (T3) and reverse T3 (rT3) were examined in the serum. The curves of T4 circadian oscillations showed two peaks in all the photoperiods followed. Computative acrophases were localized between 07.00 and 08.00 h, the amplitude in the LD 12:12 regimen was twice that observed in LD 08:16 and 16:08, the rhythm was present and the mesors were approximately the same. Circadian oscillations of T3 exhibited rhythmicity in all the photoperiods with computative acrophases localized between 07.30 and 09.00 h, and the values of mesors in LD 08:16 and 16:08 regimens were significantly lower in comparison with those in the LD 12:12 regimen. The rT3 circadian variations in the LD 12:12 regimen showed rhythmicity with acrophase at 06.00 h. The rhythm in the LD 16:08 regimen was of borderline significance, the computative acrophase occurred at 8.16 h, and the mesor value was significantly higher than those in the LD 12:12 regimen. The decrease in the amplitude of T4 oscillations and the lower T3 mesors in LD 08:16 and 16:08 regimens in comparison with the LD 12:12 values indicated only minor modification in circadian oscillations of T4 and T3 resulting from artificial photoperiods. In comparison with our previous studies these data suggest that changes in circadian oscillations of serum thyroid hormones might reflect the effect of the season of the year rather than the effect of day duration, i.e. the photoperiod.