RESUMEN
The present study analyses the activity of 5'deiodinases type I and II in mammary gland during the breeding and estrous cycle of the rat, and includes indirect evidence that 5'D-I is present only in the alveolar epithelium. Data show that the mammary gland exhibits 5'D-II activity throughout the developmental period and its activity varies along the allometric growth of the gland. 5'D-I is detected during the differentiation stages of the alveolar epithelium (puberty, late pregnancy) and its activity rises significantly (10-fold) 24 h after delivery. Data also show that 5'D-I activity is not present in the fat pads of the gland. These findings suggest that during its differentiation and functional stages, the mammary gland requires an elevated and compartmentalized production of T3.