Type II 5' deiodinase (5'D-II) activity was assessed in the hypothalamus (HP), pituitary (PIT), and adrenal gland (AG) of adult male rats after two different photoperiodic regimes: (1) cyclic light-dark conditions (LD, 12∶12) or (2) 4 days in continuous darkness (DD). Under cyclic LD conditions HP, PIT and AG 5'D-11 activity showed a significant circadian rhythm (P<0.004-<0.0001). The acrophases obtained by cosinor analysis were: HP, 12∶37; PIT, 03∶04 and AG, 02∶30 h. In contrast, after 4 days in DD, HP 5'D-II rhythmicity was abolished and there was a significant shift in the acrophase of both PIT and AG enzyme activity. These results suggest that the 5'D-II oscillation in PIT and AG corresponds to an endogenous circadian rhythm and is not only a response to the presence of a L/D cycle.