Male SPF rats (Wistar strain) were adapted in the course of the year to natural light (N) and to a 12:12 h (light:dark) artificial light (A) regimen. At approximately the spring and autumn equinox and the summer and winter solstice, rats were killed at 3-h intervals over a 24 h period and their serum thyroxine (T4), triiodothyronine (T3) and reverse T3 levels were determined. The light regimen and time of year significantly influenced the basic characteristics of the oscillations of the hormones. In the N regimen, T4 levels (T3 levels less) culminated in all seasons in correlation to sunrise. In the A regimen they culminated irregularly after daybreak. In animals with the N regimen, the oscillations of the hormones were rhythmic in all seasons, but in the A regimen in only some seasons. In the N regimen, the mean daily T4 concentration value (the mesor) was the highest in the spring and the lowest in the autumn; in the A regimen the mesors were the same, except for a low mesor in the autumn. In both light regimens, the T3 mesors were the highest in the autumn and low in the winter; the rT3 mesors were a mirror image of the T3 mesors. The annual mean of serum T4 concentrations was lower in the N group than in the A group.