The presence of isoenzymes mediating the conversion of thyroxine to 3,5,3'-triiodothyronine has been studied according to characteristic kinetics and physiological regulation. In this paper, we report the expression of type II 5'-deiodinase (5'D) activity in oocytes of Xenopus laevis. Oocytes injected with total RNA extracted from rat Harderian gland, and then incubated up to five days demonstrated a progressive increase in 5'D activity, reaching a maximal value at 24 h; then, 5'D activity remained almost stable for an additional period of four days. Characteristics of the enzyme activity expressed by oocytes included its inhibition by iopanoic acid, but not by propylthiouracil, and its increase during beta-adrenergic agonist treatment and hypothyroidism. The expressed activity manifests characteristics typical of the type II isoenzyme. Deiodinating activity in oocytes also exhibited diurnal variations. In this study, 5'D activity expressed in oocytes exhibited low values when animals were killed during the day, and high values when animals were killed at night. Maximal values were reached 3-4 h before the nocturnal peak of 5'D activity in Harderian gland crude homogenates. Results suggest that the in vivo activation of 5'D by isoproterenol, hypothyroidism, or dark exposure may be caused by an increase in the synthesis and/or maturation of the RNA expressing the enzyme.